[clang] [llvm] Use XMACROS for MachO platforms. (PR #69262)

[Jonas Devlieghere via cfe-commits]

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

I like how this centralizes everything in a single place and the def file format seems like a natural fit for the platforms. 

https://github.com/llvm/llvm-project/pull/69262