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

[Zixu Wang via cfe-commits]

zixu-w wrote:

> Is the motivation for this change in behavior (on macOS) documented somewhere? Besides zlib as already discussed above, pre-defining `TARGET_OS_MAC` apparently at least also affects some libpng builds (and I ultimately had to globally set `-fno-define-target-os-macros` for now to make a test build of LibreOffice and all its included external libraries work).

I should be able to put up documentation/release note regarding the change.

IMO, we've also noticed the issue and upstreamed a fix for libpng (https://github.com/pnggroup/libpng/pull/529)

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