[libcxx-commits] [libcxx] [libc++][test] Enhance `ADDITIONAL_COMPILE_FLAGS` to work with MSVC (PR #74974)

[S. B. Tam via libcxx-commits]

cpplearner wrote:

IMHO, `TEST_CLANG_DIAGNOSTIC_IGNORED`/`TEST_GCC_DIAGNOSTIC_IGNORED`/`TEST_MSVC_DIAGNOSTIC_IGNORED` (from [test_macros.h](https://github.com/llvm/llvm-project/blob/78623b079b3be841e96ce968ae5156fe26f6c565/libcxx/test/support/test_macros.h#L408-L432)) should be the preferred way to disable warnings.

https://github.com/llvm/llvm-project/pull/74974