[clang] [clang-tools-extra] Implement Sema iosmac support (PR #218843)

[Ian Anderson via cfe-commits]

ian-twilightcoder wrote:

> Does that need an RFC?

I don't think so, it's 8 years relied upon Mac Catalyst behavior, but @ahatanak what do you think?

https://github.com/llvm/llvm-project/pull/218843