[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 `ppc64le-flang-rhel-clang` running on `ppc64le-flang-rhel-test` while building `llvm` at step 3 "clean-build-dir".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/157/builds/50921

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

```
Step 3 (clean-build-dir) failure: Delete failed. (failure) (timed out)
Step 5 (build-unified-tree) failure: build (failure) (timed out)
...
2341.873 [73/8/4823] Linking CXX executable tools/flang/unittests/Evaluate/integer.test
2342.405 [73/7/4824] Linking CXX executable tools/flang/unittests/Evaluate/real.test
2343.059 [73/6/4825] Linking CXX executable tools/flang/unittests/Evaluate/intrinsics.test
2343.092 [73/5/4826] Linking CXX executable tools/flang/unittests/Evaluate/expression.test
2343.619 [73/4/4827] Linking CXX executable tools/flang/unittests/Evaluate/folding.test
2424.786 [73/3/4828] Building CXX object tools/flang/lib/Frontend/CMakeFiles/flangFrontend.dir/ParserActions.cpp.o
2530.536 [73/2/4829] Building CXX object tools/flang/lib/Lower/CMakeFiles/FortranLower.dir/OpenMP/OpenMP.cpp.o
2531.848 [72/2/4830] Linking CXX static library lib/libFortranLower.a
2532.519 [71/2/4831] Linking CXX static library lib/libflangFrontend.a
2550.041 [70/2/4832] Linking CXX executable bin/bbc
command timed out: 1200 seconds without output running [b'ninja'], attempting to kill
process killed by signal 9
program finished with exit code -1
elapsedTime=3752.008554

```

</details>

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