[clang-tools-extra] [llvm] [llvm] add support for mustache templating language (PR #105893)

[LLVM Continuous Integration via cfe-commits]

llvm-ci wrote:

LLVM Buildbot has detected a new failure on builder `clang-s390x-linux-lnt` running on `systemz-1` while building `llvm` at step 6 "build stage 1".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/136/builds/3100

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

```
Step 6 (build stage 1) failure: 'ninja -j4' (failure)
...
[317/6002] Copying clang-doc-default-stylesheet.css to /home/uweigand/sandbox/buildbot/clang-s390x-linux-lnt/stage1/./bin/../share/clang-doc
[318/6002] Generating ../../../../share/scan-view/startfile.py
[319/6002] Copying index.js to /home/uweigand/sandbox/buildbot/clang-s390x-linux-lnt/stage1/./bin/../share/clang-doc
[320/6002] Building CXX object tools/clang/tools/extra/clang-tidy/misc/ConfusableTable/CMakeFiles/clang-tidy-confusable-chars-gen.dir/BuildConfusableTable.cpp.o
[321/6002] Building CXX object tools/llvm-undname/CMakeFiles/llvm-undname.dir/llvm-undname.cpp.o
[322/6002] Building CXX object tools/reduce-chunk-list/CMakeFiles/reduce-chunk-list.dir/reduce-chunk-list.cpp.o
[323/6002] Building CXX object unittests/Support/DynamicLibrary/CMakeFiles/DynamicLibraryLib.dir/ExportedFuncs.cpp.o
[324/6002] Building CXX object unittests/Support/DynamicLibrary/CMakeFiles/PipSqueak.dir/PipSqueak.cpp.o
[325/6002] Creating export file for Remarks
[326/6002] Building CXX object lib/Support/CMakeFiles/LLVMSupport.dir/Mustache.cpp.o
FAILED: lib/Support/CMakeFiles/LLVMSupport.dir/Mustache.cpp.o 
CCACHE_CPP2=yes CCACHE_HASHDIR=yes /usr/bin/ccache /usr/bin/c++ -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/home/uweigand/sandbox/buildbot/clang-s390x-linux-lnt/stage1/lib/Support -I/home/uweigand/sandbox/buildbot/clang-s390x-linux-lnt/llvm/llvm/lib/Support -I/home/uweigand/sandbox/buildbot/clang-s390x-linux-lnt/stage1/include -I/home/uweigand/sandbox/buildbot/clang-s390x-linux-lnt/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-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 -O3 -DNDEBUG -UNDEBUG -std=c++17  -fno-exceptions -funwind-tables -fno-rtti -MD -MT lib/Support/CMakeFiles/LLVMSupport.dir/Mustache.cpp.o -MF lib/Support/CMakeFiles/LLVMSupport.dir/Mustache.cpp.o.d -o lib/Support/CMakeFiles/LLVMSupport.dir/Mustache.cpp.o -c /home/uweigand/sandbox/buildbot/clang-s390x-linux-lnt/llvm/llvm/lib/Support/Mustache.cpp
/home/uweigand/sandbox/buildbot/clang-s390x-linux-lnt/llvm/llvm/lib/Support/Mustache.cpp:109:12: error: declaration of '{anonymous}::Accessor llvm::mustache::Token::Accessor' changes meaning of 'Accessor' [-fpermissive]
  109 |   Accessor Accessor;
      |            ^~~~~~~~
/home/uweigand/sandbox/buildbot/clang-s390x-linux-lnt/llvm/llvm/lib/Support/Mustache.cpp:19:7: note: 'Accessor' declared here as 'using Accessor = class llvm::SmallVector<std::__cxx11::basic_string<char> >'
   19 | using Accessor = SmallVector<std::string>;
      |       ^~~~~~~~
/home/uweigand/sandbox/buildbot/clang-s390x-linux-lnt/llvm/llvm/lib/Support/Mustache.cpp:186:18: error: declaration of 'const Accessor llvm::mustache::ASTNode::Accessor' changes meaning of 'Accessor' [-fpermissive]
  186 |   const Accessor Accessor;
      |                  ^~~~~~~~
/home/uweigand/sandbox/buildbot/clang-s390x-linux-lnt/llvm/llvm/lib/Support/Mustache.cpp:19:7: note: 'Accessor' declared here as 'using Accessor = class llvm::SmallVector<std::__cxx11::basic_string<char> >'
   19 | using Accessor = SmallVector<std::string>;
      |       ^~~~~~~~
In file included from /home/uweigand/sandbox/buildbot/clang-s390x-linux-lnt/llvm/llvm/lib/Support/Mustache.cpp:8:
/home/uweigand/sandbox/buildbot/clang-s390x-linux-lnt/llvm/llvm/include/llvm/Support/Mustache.h: In constructor 'llvm::mustache::Template::Template(llvm::mustache::Template&&)':
/home/uweigand/sandbox/buildbot/clang-s390x-linux-lnt/llvm/llvm/include/llvm/Support/Mustache.h:123:12: warning: 'llvm::mustache::Template::Tree' will be initialized after [-Wreorder]
  123 |   ASTNode *Tree;
      |            ^~~~
/home/uweigand/sandbox/buildbot/clang-s390x-linux-lnt/llvm/llvm/include/llvm/Support/Mustache.h:120:26: warning:   'llvm::BumpPtrAllocator llvm::mustache::Template::AstAllocator' [-Wreorder]
  120 |   llvm::BumpPtrAllocator AstAllocator;
      |                          ^~~~~~~~~~~~
/home/uweigand/sandbox/buildbot/clang-s390x-linux-lnt/llvm/llvm/lib/Support/Mustache.cpp:769:1: warning:   when initialized here [-Wreorder]
  769 | Template::Template(Template &&Other) noexcept
      | ^~~~~~~~
[327/6002] Linking CXX static library lib/libDynamicLibraryLib.a
[328/6002] Building CXX object unittests/Support/DynamicLibrary/CMakeFiles/SecondLib.dir/PipSqueak.cpp.o
[329/6002] Linking CXX shared module unittests/Support/DynamicLibrary/PipSqueak.so
ninja: build stopped: subcommand failed.

```

</details>

https://github.com/llvm/llvm-project/pull/105893