[clang] [C++26][clang] Implement P2795R5 'Erroneous behaviour for uninitialized reads' (PR #177614)

[via cfe-commits]

github-actions[bot] wrote:

<!--PREMERGE ADVISOR COMMENT: Linux-->
# :penguin: Linux x64 Test Results

The build failed before running any tests. Click on a failure below to see the details.

<details>
<summary>tools/clang/lib/AST/CMakeFiles/obj.clangAST.dir/APValue.cpp.o</summary>

```
FAILED: tools/clang/lib/AST/CMakeFiles/obj.clangAST.dir/APValue.cpp.o
sccache /opt/llvm/bin/clang++ -DCLANG_EXPORTS -D_DEBUG -D_GLIBCXX_ASSERTIONS -D_GLIBCXX_USE_CXX11_ABI=1 -D_GNU_SOURCE -D__STDC_CONSTANT_MACROS -D__STDC_FORMAT_MACROS -D__STDC_LIMIT_MACROS -I/home/gha/actions-runner/_work/llvm-project/llvm-project/build/tools/clang/lib/AST -I/home/gha/actions-runner/_work/llvm-project/llvm-project/clang/lib/AST -I/home/gha/actions-runner/_work/llvm-project/llvm-project/clang/include -I/home/gha/actions-runner/_work/llvm-project/llvm-project/build/tools/clang/include -I/home/gha/actions-runner/_work/llvm-project/llvm-project/build/include -I/home/gha/actions-runner/_work/llvm-project/llvm-project/llvm/include -gmlt -fPIC -fno-semantic-interposition -fvisibility-inlines-hidden -Werror -Werror=date-time -Werror=unguarded-availability-new -Wall -Wextra -Wno-unused-parameter -Wwrite-strings -Wcast-qual -Wmissing-field-initializers -pedantic -Wno-long-long -Wc++98-compat-extra-semi -Wimplicit-fallthrough -Wcovered-switch-default -Wno-noexcept-type -Wnon-virtual-dtor -Wdelete-non-virtual-dtor -Wsuggest-override -Wstring-conversion -Wno-pass-failed -Wmisleading-indentation -Wctad-maybe-unsupported -fdiagnostics-color -ffunction-sections -fdata-sections -fno-common -Woverloaded-virtual -Wno-nested-anon-types -O3 -DNDEBUG -std=c++17 -UNDEBUG -fno-exceptions -funwind-tables -fno-rtti -MD -MT tools/clang/lib/AST/CMakeFiles/obj.clangAST.dir/APValue.cpp.o -MF tools/clang/lib/AST/CMakeFiles/obj.clangAST.dir/APValue.cpp.o.d -o tools/clang/lib/AST/CMakeFiles/obj.clangAST.dir/APValue.cpp.o -c /home/gha/actions-runner/_work/llvm-project/llvm-project/clang/lib/AST/APValue.cpp
/home/gha/actions-runner/_work/llvm-project/llvm-project/clang/lib/AST/APValue.cpp:313:11: error: enumeration value 'Erroneous' not handled in switch [-Werror,-Wswitch]
313 |   switch (RHS.getKind()) {
|           ^~~~~~~~~~~~~
/home/gha/actions-runner/_work/llvm-project/llvm-project/clang/lib/AST/APValue.cpp:438:11: error: enumeration value 'Erroneous' not handled in switch [-Werror,-Wswitch]
438 |   switch (getKind()) {
|           ^~~~~~~~~
/home/gha/actions-runner/_work/llvm-project/llvm-project/clang/lib/AST/APValue.cpp:497:11: error: enumeration value 'Erroneous' not handled in switch [-Werror,-Wswitch]
497 |   switch (Kind) {
|           ^~~~
/home/gha/actions-runner/_work/llvm-project/llvm-project/clang/lib/AST/APValue.cpp:729:3: error: unannotated fall-through between switch labels [-Werror,-Wimplicit-fallthrough]
729 |   case APValue::Int:
|   ^
/home/gha/actions-runner/_work/llvm-project/llvm-project/clang/lib/AST/APValue.cpp:729:3: note: insert 'LLVM_FALLTHROUGH;' to silence this warning
729 |   case APValue::Int:
|   ^
|   LLVM_FALLTHROUGH;
/home/gha/actions-runner/_work/llvm-project/llvm-project/clang/lib/AST/APValue.cpp:729:3: note: insert 'break;' to avoid fall-through
729 |   case APValue::Int:
|   ^
|   break;
4 errors generated.
```
</details>

If these failures are unrelated to your changes (for example tests are broken or flaky at HEAD), please open an issue at https://github.com/llvm/llvm-project/issues and add the `infrastructure` label.

https://github.com/llvm/llvm-project/pull/177614