[llvm] IR: Introduce "thread-model" module flag (PR #222768)

[LLVM Continuous Integration via llvm-commits]

llvm-ci wrote:

LLVM Buildbot has detected a new failure on builder `clang-x86_64-linux-abi-test` running on `sie-linux-worker2` while building `llvm` at step 6 "build-unified-tree".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/8/builds/34447

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

```
Step 6 (build-unified-tree) failure: build (failure)
...
6297.062 [1495/10/6484] Building CXX object tools/clang/unittests/CMakeFiles/AllClangUnitTests.dir/ASTMatchers/ASTMatchersNodeTest.cpp.o
6305.551 [1494/10/6485] Building CXX object tools/clang/unittests/CMakeFiles/AllClangUnitTests.dir/AST/ASTImporterFixtures.cpp.o
6306.783 [1493/10/6486] Building CXX object tools/clang/unittests/CMakeFiles/AllClangUnitTests.dir/AST/ASTExprTest.cpp.o
6312.045 [1492/10/6487] Building CXX object tools/clang/unittests/CMakeFiles/AllClangUnitTests.dir/ASTMatchers/ASTMatchersNarrowingTest.cpp.o
6312.261 [1491/10/6488] Building CXX object tools/clang/unittests/CMakeFiles/AllClangUnitTests.dir/AST/ASTImporterObjCTest.cpp.o
6312.311 [1490/10/6489] Building CXX object tools/clang/unittests/CMakeFiles/AllClangUnitTests.dir/AST/CommentLexer.cpp.o
6315.708 [1489/10/6490] Building CXX object tools/clang/unittests/CMakeFiles/AllClangUnitTests.dir/AST/ASTVectorTest.cpp.o
6315.763 [1488/10/6491] Building CXX object tools/clang/unittests/CMakeFiles/AllClangUnitTests.dir/AST/CommentTextTest.cpp.o
6326.927 [1487/10/6492] Building CXX object tools/clang/unittests/CMakeFiles/AllClangUnitTests.dir/AST/CommentParser.cpp.o
6411.497 [1486/10/6493] Building CXX object tools/clang/unittests/CMakeFiles/AllClangUnitTests.dir/ASTMatchers/ASTMatchersTraversalTest.cpp.o
FAILED: tools/clang/unittests/CMakeFiles/AllClangUnitTests.dir/ASTMatchers/ASTMatchersTraversalTest.cpp.o 
/opt/ccache/bin/g++ -DLLVM_BUILD_STATIC -D_DEBUG -D_GLIBCXX_ASSERTIONS -D_GLIBCXX_USE_CXX11_ABI=1 -D_GNU_SOURCE -D_LIBCPP_HARDENING_MODE=_LIBCPP_HARDENING_MODE_EXTENSIVE -D__STDC_CONSTANT_MACROS -D__STDC_FORMAT_MACROS -D__STDC_LIMIT_MACROS -I/home/buildbot/buildbot-root/abi-test/build/tools/clang/unittests -I/home/buildbot/buildbot-root/abi-test/llvm/clang/unittests -I/home/buildbot/buildbot-root/abi-test/llvm/clang/include -I/home/buildbot/buildbot-root/abi-test/build/tools/clang/include -I/home/buildbot/buildbot-root/abi-test/build/include -I/home/buildbot/buildbot-root/abi-test/llvm/llvm/include -I/home/buildbot/buildbot-root/abi-test/llvm/clang/unittests/Tooling -I/home/buildbot/buildbot-root/abi-test/llvm/third-party/unittest/googletest/include -I/home/buildbot/buildbot-root/abi-test/llvm/third-party/unittest/googlemock/include -fPIC -fno-semantic-interposition -fvisibility-inlines-hidden -Werror=date-time -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-array-bounds -Wno-stringop-overread -Wno-unused-but-set-parameter -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 -O3 -DNDEBUG -UNDEBUG -Wno-variadic-macros -fno-exceptions -funwind-tables -fno-rtti -Wno-suggest-override -std=c++17 -MD -MT tools/clang/unittests/CMakeFiles/AllClangUnitTests.dir/ASTMatchers/ASTMatchersTraversalTest.cpp.o -MF tools/clang/unittests/CMakeFiles/AllClangUnitTests.dir/ASTMatchers/ASTMatchersTraversalTest.cpp.o.d -o tools/clang/unittests/CMakeFiles/AllClangUnitTests.dir/ASTMatchers/ASTMatchersTraversalTest.cpp.o -c /home/buildbot/buildbot-root/abi-test/llvm/clang/unittests/ASTMatchers/ASTMatchersTraversalTest.cpp
g++: fatal error: Killed signal terminated program cc1plus
compilation terminated.
6414.171 [1486/9/6494] Building CXX object tools/clang/unittests/CMakeFiles/AllClangUnitTests.dir/AST/ASTTypeTraitsTest.cpp.o
6416.245 [1486/8/6495] Building CXX object tools/clang/unittests/CMakeFiles/AllClangUnitTests.dir/AST/ASTTraverserTest.cpp.o
6416.863 [1486/7/6496] Building CXX object tools/clang/unittests/CMakeFiles/AllClangUnitTests.dir/AST/ASTImporterGenericRedeclTest.cpp.o
6417.336 [1486/6/6497] Building CXX object tools/clang/unittests/CMakeFiles/AllClangUnitTests.dir/AST/ASTImporterVisibilityTest.cpp.o
6417.526 [1486/5/6498] Building CXX object tools/clang/unittests/CMakeFiles/AllClangUnitTests.dir/AST/AttrTest.cpp.o
6418.423 [1486/4/6499] Building CXX object tools/clang/unittests/CMakeFiles/AllClangUnitTests.dir/AST/ASTImporterODRStrategiesTest.cpp.o
6418.958 [1486/3/6500] Building CXX object tools/clang/unittests/CMakeFiles/AllClangUnitTests.dir/AST/ConceptPrinterTest.cpp.o
6425.955 [1486/2/6501] Building CXX object tools/clang/unittests/CMakeFiles/AllClangUnitTests.dir/AST/DataCollectionTest.cpp.o
6450.951 [1486/1/6502] Building CXX object tools/clang/unittests/CMakeFiles/AllClangUnitTests.dir/AST/ASTImporterTest.cpp.o
ninja: build stopped: subcommand failed.

```

</details>

https://github.com/llvm/llvm-project/pull/222768