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

[via cfe-commits]

davidtrevelyan wrote:

@cjappl for review (and please could you also request reviews from the key interested parties?)

PS I'll apply for membership after this PR so I hope this is the last time you'll have to do this for me :)

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