[llvm] [docs] Add guide for Undefined Behavior (PR #119220)

[John Regehr via llvm-commits]

regehr wrote:

Nuno this is great and I don't know why we didn't do it like >5 years ago. 

Maybe link to the "taming undefined behavior" paper, for people who want more gory details?

Maybe we should include a bunch of links into our CE instance so that people can play with this stuff and see how refinement checks fall out in practice? Of course this shouldn't block this from getting added, we can add stuff later, and I'm happy to help.

Also I think we can write quite a bit more in the "writing tests that avoid UB" part.

But anyway I think this adds value as-is and there's no reason not to merge it, then keep working.

https://github.com/llvm/llvm-project/pull/119220