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

[Fangrui Song via llvm-commits]

MaskRay wrote:

I'll take a quick look at this patch today, but probably more focused on the style.

For easy reference, the description/commit message should link to the RFC discussion: https://discourse.llvm.org/t/rfc-revival-of-numerical-sanitizer/79601
People have high-level suggestions are recommended to comment there.

**There has been significant offline interest in this revival.**
I have chatted with @alexander-shaposhnikov, who mentioned that several companies and organizations expressing enthusiasm.
I personally mentioned this effort last December to a few folks working on floating-point arithmetic for C library functions,
and they're interested in the feature. While they support the idea, building llvm-project isn't familiar territory for them, and they'd prefer having the code in the tree for further suggestions.

However, **there's room for improvement in the process**. 
Ideally, future RFCs could be posted sooner to generate early discussion. While this revival builds on @legrosbuffle's well-supported earlier work (https://reviews.llvm.org/D97854), the delay highlights the need for a streamlined process for future revivals.
To better gauge industry interest and facilitate collaboration, the discourse post is a place to explicitly record support from companies and organizations in the RFC process.

In light of recent proposals involving instrumentation and compiler-rt, we're refining the RFC process to establish clearer acceptance criteria.


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