[compiler-rt] [compiler-rt][nsan] Add support for nan detection (PR #101531)

[Alexander Shaposhnikov via llvm-commits]

https://github.com/alexander-shaposhnikov approved this pull request.

Added a few comments, 
otherwise, LGTM.
We can merge it in,
I'll probably do some cleanup afterwards.

https://github.com/llvm/llvm-project/pull/101531