[llvm] [LLVM][Instrumentation] Add numerical sanitizer (PR #85916)

[Fangrui Song via llvm-commits]

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

I made a few nits, then I realized that I can just publish https://github.com/MaskRay/llvm-project/tree/nsan The top commit contains code style changes.

https://github.com/llvm/llvm-project/pull/85916