[clang] [C2y] Add test coverage and documentation for WG14 N3342 (PR #115494)

[LLVM Continuous Integration via cfe-commits]

llvm-ci wrote:

LLVM Buildbot has detected a new failure on builder `clang-m68k-linux-cross` running on `suse-gary-m68k-cross` while building `clang` at step 4 "build stage 1".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/27/builds/1746

<details>
<summary>Here is the relevant piece of the build log for the reference</summary>

```
Step 4 (build stage 1) failure: 'ninja' (failure)
...
In file included from /var/lib/buildbot/workers/suse-gary-m68k-cross/clang-m68k-linux-cross/llvm/clang/include/clang/Serialization/ASTReader.h:30,
                 from /var/lib/buildbot/workers/suse-gary-m68k-cross/clang-m68k-linux-cross/llvm/clang/include/clang/Serialization/ASTRecordReader.h:20,
                 from /var/lib/buildbot/workers/suse-gary-m68k-cross/clang-m68k-linux-cross/llvm/clang/lib/Serialization/ASTReaderDecl.cpp:55:
/var/lib/buildbot/workers/suse-gary-m68k-cross/clang-m68k-linux-cross/llvm/clang/include/clang/Sema/Sema.h:463:7: warning: ‘clang::Sema’ declared with greater visibility than the type of its field ‘clang::Sema::UnusedFileScopedDecls’ [-Wattributes]
  463 | class Sema final : public SemaBase {
      |       ^~~~
/var/lib/buildbot/workers/suse-gary-m68k-cross/clang-m68k-linux-cross/llvm/clang/include/clang/Sema/Sema.h:463:7: warning: ‘clang::Sema’ declared with greater visibility than the type of its field ‘clang::Sema::TentativeDefinitions’ [-Wattributes]
/var/lib/buildbot/workers/suse-gary-m68k-cross/clang-m68k-linux-cross/llvm/clang/include/clang/Sema/Sema.h:463:7: warning: ‘clang::Sema’ declared with greater visibility than the type of its field ‘clang::Sema::ExtVectorDecls’ [-Wattributes]
/var/lib/buildbot/workers/suse-gary-m68k-cross/clang-m68k-linux-cross/llvm/clang/include/clang/Sema/Sema.h:463:7: warning: ‘clang::Sema’ declared with greater visibility than the type of its field ‘clang::Sema::DelegatingCtorDecls’ [-Wattributes]
[133/328] Building CXX object tools/clang/lib/Sema/CMakeFiles/obj.clangSema.dir/SemaExpr.cpp.o
FAILED: tools/clang/lib/Sema/CMakeFiles/obj.clangSema.dir/SemaExpr.cpp.o 
/usr/bin/c++ -DCLANG_EXPORTS -DGTEST_HAS_RTTI=0 -D_DEBUG -D_GLIBCXX_ASSERTIONS -D_GNU_SOURCE -D__STDC_CONSTANT_MACROS -D__STDC_FORMAT_MACROS -D__STDC_LIMIT_MACROS -I/var/lib/buildbot/workers/suse-gary-m68k-cross/clang-m68k-linux-cross/stage1/tools/clang/lib/Sema -I/var/lib/buildbot/workers/suse-gary-m68k-cross/clang-m68k-linux-cross/llvm/clang/lib/Sema -I/var/lib/buildbot/workers/suse-gary-m68k-cross/clang-m68k-linux-cross/llvm/clang/include -I/var/lib/buildbot/workers/suse-gary-m68k-cross/clang-m68k-linux-cross/stage1/tools/clang/include -I/var/lib/buildbot/workers/suse-gary-m68k-cross/clang-m68k-linux-cross/stage1/include -I/var/lib/buildbot/workers/suse-gary-m68k-cross/clang-m68k-linux-cross/llvm/llvm/include -fPIC -fno-semantic-interposition -fvisibility-inlines-hidden -Werror=date-time -fno-lifetime-dse -Wall -Wextra -Wno-unused-parameter -Wwrite-strings -Wcast-qual -Wno-missing-field-initializers -pedantic -Wno-long-long -Wimplicit-fallthrough -Wno-maybe-uninitialized -Wno-nonnull -Wno-class-memaccess -Wno-redundant-move -Wno-pessimizing-move -Wno-noexcept-type -Wdelete-non-virtual-dtor -Wsuggest-override -Wno-comment -Wno-misleading-indentation -Wctad-maybe-unsupported -fdiagnostics-color -ffunction-sections -fdata-sections -fno-common -Woverloaded-virtual -fno-strict-aliasing -O3 -DNDEBUG -std=c++17  -fno-exceptions -funwind-tables -fno-rtti -UNDEBUG -MD -MT tools/clang/lib/Sema/CMakeFiles/obj.clangSema.dir/SemaExpr.cpp.o -MF tools/clang/lib/Sema/CMakeFiles/obj.clangSema.dir/SemaExpr.cpp.o.d -o tools/clang/lib/Sema/CMakeFiles/obj.clangSema.dir/SemaExpr.cpp.o -c /var/lib/buildbot/workers/suse-gary-m68k-cross/clang-m68k-linux-cross/llvm/clang/lib/Sema/SemaExpr.cpp
In file included from /var/lib/buildbot/workers/suse-gary-m68k-cross/clang-m68k-linux-cross/llvm/clang/lib/Sema/CheckExprLifetime.h:17,
                 from /var/lib/buildbot/workers/suse-gary-m68k-cross/clang-m68k-linux-cross/llvm/clang/lib/Sema/SemaExpr.cpp:13:
/var/lib/buildbot/workers/suse-gary-m68k-cross/clang-m68k-linux-cross/llvm/clang/include/clang/Sema/Sema.h:463:7: warning: ‘clang::Sema’ declared with greater visibility than the type of its field ‘clang::Sema::UnusedFileScopedDecls’ [-Wattributes]
  463 | class Sema final : public SemaBase {
      |       ^~~~
/var/lib/buildbot/workers/suse-gary-m68k-cross/clang-m68k-linux-cross/llvm/clang/include/clang/Sema/Sema.h:463:7: warning: ‘clang::Sema’ declared with greater visibility than the type of its field ‘clang::Sema::TentativeDefinitions’ [-Wattributes]
/var/lib/buildbot/workers/suse-gary-m68k-cross/clang-m68k-linux-cross/llvm/clang/include/clang/Sema/Sema.h:463:7: warning: ‘clang::Sema’ declared with greater visibility than the type of its field ‘clang::Sema::ExtVectorDecls’ [-Wattributes]
/var/lib/buildbot/workers/suse-gary-m68k-cross/clang-m68k-linux-cross/llvm/clang/include/clang/Sema/Sema.h:463:7: warning: ‘clang::Sema’ declared with greater visibility than the type of its field ‘clang::Sema::DelegatingCtorDecls’ [-Wattributes]
c++: fatal error: Killed signal terminated program cc1plus
compilation terminated.
[134/328] Building CXX object tools/clang/lib/CodeGen/CMakeFiles/obj.clangCodeGen.dir/CGException.cpp.o
[135/328] Building CXX object tools/clang/lib/Frontend/CMakeFiles/obj.clangFrontend.dir/InterfaceStubFunctionsConsumer.cpp.o
In file included from /var/lib/buildbot/workers/suse-gary-m68k-cross/clang-m68k-linux-cross/llvm/clang/include/clang/Sema/TemplateInstCallback.h:17,
                 from /var/lib/buildbot/workers/suse-gary-m68k-cross/clang-m68k-linux-cross/llvm/clang/lib/Frontend/InterfaceStubFunctionsConsumer.cpp:14:
/var/lib/buildbot/workers/suse-gary-m68k-cross/clang-m68k-linux-cross/llvm/clang/include/clang/Sema/Sema.h:463:7: warning: ‘clang::Sema’ declared with greater visibility than the type of its field ‘clang::Sema::UnusedFileScopedDecls’ [-Wattributes]
  463 | class Sema final : public SemaBase {
      |       ^~~~
/var/lib/buildbot/workers/suse-gary-m68k-cross/clang-m68k-linux-cross/llvm/clang/include/clang/Sema/Sema.h:463:7: warning: ‘clang::Sema’ declared with greater visibility than the type of its field ‘clang::Sema::TentativeDefinitions’ [-Wattributes]
/var/lib/buildbot/workers/suse-gary-m68k-cross/clang-m68k-linux-cross/llvm/clang/include/clang/Sema/Sema.h:463:7: warning: ‘clang::Sema’ declared with greater visibility than the type of its field ‘clang::Sema::ExtVectorDecls’ [-Wattributes]
/var/lib/buildbot/workers/suse-gary-m68k-cross/clang-m68k-linux-cross/llvm/clang/include/clang/Sema/Sema.h:463:7: warning: ‘clang::Sema’ declared with greater visibility than the type of its field ‘clang::Sema::DelegatingCtorDecls’ [-Wattributes]
[136/328] Building CXX object tools/clang/lib/Tooling/DependencyScanning/CMakeFiles/obj.clangDependencyScanning.dir/ModuleDepCollector.cpp.o
In file included from /var/lib/buildbot/workers/suse-gary-m68k-cross/clang-m68k-linux-cross/llvm/clang/include/clang/Serialization/ASTReader.h:30,
                 from /var/lib/buildbot/workers/suse-gary-m68k-cross/clang-m68k-linux-cross/llvm/clang/include/clang/Tooling/DependencyScanning/ModuleDepCollector.h:19,
                 from /var/lib/buildbot/workers/suse-gary-m68k-cross/clang-m68k-linux-cross/llvm/clang/lib/Tooling/DependencyScanning/ModuleDepCollector.cpp:9:
/var/lib/buildbot/workers/suse-gary-m68k-cross/clang-m68k-linux-cross/llvm/clang/include/clang/Sema/Sema.h:463:7: warning: ‘clang::Sema’ declared with greater visibility than the type of its field ‘clang::Sema::UnusedFileScopedDecls’ [-Wattributes]
  463 | class Sema final : public SemaBase {
      |       ^~~~
/var/lib/buildbot/workers/suse-gary-m68k-cross/clang-m68k-linux-cross/llvm/clang/include/clang/Sema/Sema.h:463:7: warning: ‘clang::Sema’ declared with greater visibility than the type of its field ‘clang::Sema::TentativeDefinitions’ [-Wattributes]
/var/lib/buildbot/workers/suse-gary-m68k-cross/clang-m68k-linux-cross/llvm/clang/include/clang/Sema/Sema.h:463:7: warning: ‘clang::Sema’ declared with greater visibility than the type of its field ‘clang::Sema::ExtVectorDecls’ [-Wattributes]
/var/lib/buildbot/workers/suse-gary-m68k-cross/clang-m68k-linux-cross/llvm/clang/include/clang/Sema/Sema.h:463:7: warning: ‘clang::Sema’ declared with greater visibility than the type of its field ‘clang::Sema::DelegatingCtorDecls’ [-Wattributes]
[137/328] Building CXX object tools/clang/lib/Tooling/DependencyScanning/CMakeFiles/obj.clangDependencyScanning.dir/DependencyScanningWorker.cpp.o
In file included from /var/lib/buildbot/workers/suse-gary-m68k-cross/clang-m68k-linux-cross/llvm/clang/include/clang/Serialization/ASTReader.h:30,
                 from /var/lib/buildbot/workers/suse-gary-m68k-cross/clang-m68k-linux-cross/llvm/clang/include/clang/Tooling/DependencyScanning/ModuleDepCollector.h:19,
                 from /var/lib/buildbot/workers/suse-gary-m68k-cross/clang-m68k-linux-cross/llvm/clang/include/clang/Tooling/DependencyScanning/DependencyScanningWorker.h:17,
                 from /var/lib/buildbot/workers/suse-gary-m68k-cross/clang-m68k-linux-cross/llvm/clang/lib/Tooling/DependencyScanning/DependencyScanningWorker.cpp:9:
/var/lib/buildbot/workers/suse-gary-m68k-cross/clang-m68k-linux-cross/llvm/clang/include/clang/Sema/Sema.h:463:7: warning: ‘clang::Sema’ declared with greater visibility than the type of its field ‘clang::Sema::UnusedFileScopedDecls’ [-Wattributes]
  463 | class Sema final : public SemaBase {

```

</details>

https://github.com/llvm/llvm-project/pull/115494