[llvm] Add a pass to collect dropped var stats for MIR. (PR #115566)

[LLVM Continuous Integration via llvm-commits]

llvm-ci wrote:

LLVM Buildbot has detected a new failure on builder `clang-x86_64-debian-fast` running on `gribozavr4` while building `llvm` at step 6 "test-build-unified-tree-check-all".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/56/builds/12495

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

```
Step 6 (test-build-unified-tree-check-all) failure: test (failure)
...
In file included from /b/1/clang-x86_64-debian-fast/llvm.src/llvm/include/llvm/Target/TargetMachine.h:19:
/b/1/clang-x86_64-debian-fast/llvm.src/llvm/include/llvm/MC/MCStreamer.h:614:14: warning: parameter 'Sym' not found in the function declaration [-Wdocumentation]
  /// \param Sym - The symbol on the .ref directive.
             ^~~
/b/1/clang-x86_64-debian-fast/llvm.src/llvm/include/llvm/MC/MCStreamer.h:614:14: note: did you mean 'Symbol'?
  /// \param Sym - The symbol on the .ref directive.
             ^~~
             Symbol
2 warnings generated.
13.328 [17/63/1118] Building CXX object unittests/MIR/CMakeFiles/MIRTests.dir/DroppedVariableStatsMIRTest.cpp.o
FAILED: unittests/MIR/CMakeFiles/MIRTests.dir/DroppedVariableStatsMIRTest.cpp.o 
CCACHE_CPP2=yes CCACHE_HASHDIR=yes /usr/bin/ccache /usr/bin/clang++ -DGTEST_HAS_RTTI=0 -DLLVM_BUILD_STATIC -D_DEBUG -D_GLIBCXX_ASSERTIONS -D_GNU_SOURCE -D__STDC_CONSTANT_MACROS -D__STDC_FORMAT_MACROS -D__STDC_LIMIT_MACROS -I/b/1/clang-x86_64-debian-fast/llvm.obj/unittests/MIR -I/b/1/clang-x86_64-debian-fast/llvm.src/llvm/unittests/MIR -I/b/1/clang-x86_64-debian-fast/llvm.obj/include -I/b/1/clang-x86_64-debian-fast/llvm.src/llvm/include -I/b/1/clang-x86_64-debian-fast/llvm.src/third-party/unittest/googletest/include -I/b/1/clang-x86_64-debian-fast/llvm.src/third-party/unittest/googlemock/include -std=c++11 -Wdocumentation -Wno-documentation-deprecated-sync -fPIC -fvisibility-inlines-hidden -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 -Wmisleading-indentation -Wctad-maybe-unsupported -fdiagnostics-color -ffunction-sections -fdata-sections -O3 -DNDEBUG  -Wno-variadic-macros -Wno-gnu-zero-variadic-macro-arguments -fno-exceptions -funwind-tables -fno-rtti -UNDEBUG -Wno-suggest-override -std=c++17 -MD -MT unittests/MIR/CMakeFiles/MIRTests.dir/DroppedVariableStatsMIRTest.cpp.o -MF unittests/MIR/CMakeFiles/MIRTests.dir/DroppedVariableStatsMIRTest.cpp.o.d -o unittests/MIR/CMakeFiles/MIRTests.dir/DroppedVariableStatsMIRTest.cpp.o -c /b/1/clang-x86_64-debian-fast/llvm.src/llvm/unittests/MIR/DroppedVariableStatsMIRTest.cpp
In file included from /b/1/clang-x86_64-debian-fast/llvm.src/llvm/unittests/MIR/DroppedVariableStatsMIRTest.cpp:16:
/b/1/clang-x86_64-debian-fast/llvm.src/llvm/include/llvm/MC/TargetRegistry.h:85:12: warning: parameter 'ShowInst' not found in the function declaration [-Wdocumentation]
/// \param ShowInst - Whether to show the MCInst representation inline with
           ^~~~~~~~
In file included from /b/1/clang-x86_64-debian-fast/llvm.src/llvm/unittests/MIR/DroppedVariableStatsMIRTest.cpp:18:
In file included from /b/1/clang-x86_64-debian-fast/llvm.src/llvm/include/llvm/Passes/StandardInstrumentations.h:31:
In file included from /b/1/clang-x86_64-debian-fast/llvm.src/llvm/include/llvm/Transforms/IPO/SampleProfileProbe.h:21:
In file included from /b/1/clang-x86_64-debian-fast/llvm.src/llvm/include/llvm/ProfileData/SampleProf.h:28:
/b/1/clang-x86_64-debian-fast/llvm.src/llvm/include/llvm/ProfileData/HashKeyMap.h:43:6: warning: '\param' command used in a comment that is not attached to a function declaration [-Wdocumentation]
/// \param MapT The underlying associative container type.
     ^~~~~
/b/1/clang-x86_64-debian-fast/llvm.src/llvm/include/llvm/ProfileData/HashKeyMap.h:44:6: warning: '\param' command used in a comment that is not attached to a function declaration [-Wdocumentation]
/// \param KeyT The original key type, which requires the implementation of
     ^~~~~
/b/1/clang-x86_64-debian-fast/llvm.src/llvm/include/llvm/ProfileData/HashKeyMap.h:46:6: warning: '\param' command used in a comment that is not attached to a function declaration [-Wdocumentation]
/// \param ValueT The original mapped type, which has the same requirement as
     ^~~~~
/b/1/clang-x86_64-debian-fast/llvm.src/llvm/include/llvm/ProfileData/HashKeyMap.h:48:6: warning: '\param' command used in a comment that is not attached to a function declaration [-Wdocumentation]
/// \param MapTArgs Additional template parameters passed to the underlying
     ^~~~~
In file included from /b/1/clang-x86_64-debian-fast/llvm.src/llvm/unittests/MIR/DroppedVariableStatsMIRTest.cpp:20:
In file included from /b/1/clang-x86_64-debian-fast/llvm.src/llvm/include/llvm/Target/TargetMachine.h:19:
In file included from /b/1/clang-x86_64-debian-fast/llvm.src/llvm/include/llvm/MC/MCStreamer.h:30:
/b/1/clang-x86_64-debian-fast/llvm.src/llvm/include/llvm/TargetParser/ARMTargetParser.h:267:12: warning: parameter 'Arch' not found in the function declaration [-Wdocumentation]
/// \param Arch the architecture name (e.g., "armv7s"). If it is an empty
           ^~~~
/b/1/clang-x86_64-debian-fast/llvm.src/llvm/include/llvm/TargetParser/ARMTargetParser.h:267:12: note: did you mean 'MArch'?
/// \param Arch the architecture name (e.g., "armv7s"). If it is an empty
           ^~~~
           MArch
In file included from /b/1/clang-x86_64-debian-fast/llvm.src/llvm/unittests/MIR/DroppedVariableStatsMIRTest.cpp:20:
In file included from /b/1/clang-x86_64-debian-fast/llvm.src/llvm/include/llvm/Target/TargetMachine.h:19:
/b/1/clang-x86_64-debian-fast/llvm.src/llvm/include/llvm/MC/MCStreamer.h:614:14: warning: parameter 'Sym' not found in the function declaration [-Wdocumentation]
  /// \param Sym - The symbol on the .ref directive.
             ^~~
/b/1/clang-x86_64-debian-fast/llvm.src/llvm/include/llvm/MC/MCStreamer.h:614:14: note: did you mean 'Symbol'?
  /// \param Sym - The symbol on the .ref directive.

```

</details>

https://github.com/llvm/llvm-project/pull/115566