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

[Shafik Yaghmour via llvm-commits]

https://github.com/shafik commented:

Thank you for this write-up. I don't usually sit at this level but I do often work with bug reports dealing w/ UB where this matters.

Maybe it does not fit perfectly here, not sure, but I feel like some greater discussion about some of the more unintuitive effects of UB might be helpful.

One of them that has come up often is when function bodies end up merged due to UB e.g. https://godbolt.org/z/8sEE54E7d

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