[clang] [clang][PP] Add extension to predefine target OS macros (PR #74676)

[Florian Hahn via cfe-commits]

fhahn wrote:

It looks like this may also break both Blender and Povray. Worked around build failures in SPEC2017 in llvm-test-suite in 9b14a92d585657b87f2ca2b135ba9044685393be, but the projects will likely need to be fixed at the source.

https://github.com/llvm/llvm-project/pull/74676