[libcxx-commits] [libcxx] [libc++][test] Enhance `ADDITIONAL_COMPILE_FLAGS`, use `TEST_MEOW_DIAGNOSTIC_IGNORED` sparingly (PR #75317)

[Nikolas Klauser via libcxx-commits]

https://github.com/philnik777 approved this pull request.

LGTM, but let's wait a few days in case other people aren't happy with this.

https://github.com/llvm/llvm-project/pull/75317