[llvm] Add macOS premerge check workflow (PR #207435)

[Tom Stellard via llvm-commits]

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

LGTM. This seems fine.  I also don't mind if this just replaces the existing macOS job which was being used on the release branch.

https://github.com/llvm/llvm-project/pull/207435