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

[Nikita Popov via llvm-commits]

https://github.com/nikic commented:

I like the idea, and this looks pretty reasonable at a glance.

I think one topic that may be worth mentioning here is the "logical and/or" pattern. It's a non-obvious construction used to get the correct poison propagation semantics.

Not sure if it's out of scope, but we could also mention related APIs, like isGuaranteedNotToBePoison.

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