[clang] [Clang] Improve infrastructure for libstdc++ workarounds (PR #141977)

[LLVM Continuous Integration via cfe-commits]

llvm-ci wrote:

LLVM Buildbot has detected a new failure on builder `ppc64le-lld-multistage-test` running on `ppc64le-lld-multistage-test` while building `clang` at step 13 "test-build-stage2-unified-tree-check-all".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/168/builds/12688

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

```
Step 13 (test-build-stage2-unified-tree-check-all) failure: test (failure)
******************** TEST 'Clang :: SemaCXX/libstdcxx_common_type_hack.cpp' FAILED ********************
Exit Code: 1

Command Output (stderr):
--
/home/buildbots/llvm-external-buildbots/workers/ppc64le-lld-multistage-test/ppc64le-lld-multistage-test/build/stage2/bin/clang -cc1 -internal-isystem /home/buildbots/llvm-external-buildbots/workers/ppc64le-lld-multistage-test/ppc64le-lld-multistage-test/build/stage2/lib/clang/21/include -nostdsysteminc -fsyntax-only /home/buildbots/llvm-external-buildbots/workers/ppc64le-lld-multistage-test/ppc64le-lld-multistage-test/llvm-project/clang/test/SemaCXX/libstdcxx_common_type_hack.cpp -std=c++11 -verify -D__GLIBCXX__=20100000L # RUN: at line 1
+ /home/buildbots/llvm-external-buildbots/workers/ppc64le-lld-multistage-test/ppc64le-lld-multistage-test/build/stage2/bin/clang -cc1 -internal-isystem /home/buildbots/llvm-external-buildbots/workers/ppc64le-lld-multistage-test/ppc64le-lld-multistage-test/build/stage2/lib/clang/21/include -nostdsysteminc -fsyntax-only /home/buildbots/llvm-external-buildbots/workers/ppc64le-lld-multistage-test/ppc64le-lld-multistage-test/llvm-project/clang/test/SemaCXX/libstdcxx_common_type_hack.cpp -std=c++11 -verify -D__GLIBCXX__=20100000L
error: 'expected-error' diagnostics seen but not expected: 
  File /home/buildbots/llvm-external-buildbots/workers/ppc64le-lld-multistage-test/ppc64le-lld-multistage-test/llvm-project/clang/test/SemaCXX/libstdcxx_common_type_hack.cpp Line 28: type alias redefinition with different types ('std::common_type<int, int>::type' (aka 'int &&') vs 'int')
error: 'expected-note' diagnostics seen but not expected: 
  File /home/buildbots/llvm-external-buildbots/workers/ppc64le-lld-multistage-test/ppc64le-lld-multistage-test/llvm-project/clang/test/SemaCXX/libstdcxx_common_type_hack.cpp Line 27: previous definition is here
2 errors generated.

--

********************


```

</details>

https://github.com/llvm/llvm-project/pull/141977