[llvm] [LLVM][rtsan] Add nonblocking attribute for the realtime sanitizer (PR #100596)

[Chris Apple via llvm-commits]

cjappl wrote:

> Can't say for sure without seeing the spec, but this is almost certainly the wrong name for the attribute. `nonblocking` would imply that "blocking" (for some rigorous definition thereof) is UB and LLVM can optimize with that knowledge. What you probably want is a `sanitize_realtime` attribute or something.

This name was chosen to match the name of the function effect it keys off of. This sanitizer works hand in hand with clang "function effects" that look like `[[clang::nonblocking]]` and `[[clang::blocking]]`
https://discourse.llvm.org/t/rfc-nolock-and-noalloc-attributes/76837

A "reviewers guide" can be found here, describing how the sanitizer works:
https://github.com/realtime-sanitizer/radsan/blob/doc/review-support/doc/review.md

We will defer to people more experienced in LLVM what the appropriate choice is here!

https://github.com/llvm/llvm-project/pull/100596