[clang] [clang][CMake] Auto-detect macOS SDK for DEFAULT_SYSROOT (PR #185986)

[via cfe-commits]

Noerr wrote:

CC potential reviewers:                                                                                                  
  - @nico — reverted the original runtime approach, already engaged on #137352                    
  - @petrhosek — CMake maintainer                                                                 
  - @MaskRay — Driver maintainer

https://github.com/llvm/llvm-project/pull/185986