[clang] [clang][rtsan] Add sanitize_realtime_unsafe attr to [[clang::blocking]] function IR (PR #111055)

[Chris Apple via cfe-commits]

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

Will wait to merge for a bit to let others chime in, but LGTM

https://github.com/llvm/llvm-project/pull/111055