[llvm] LangRef: mention that willreturn can cause time-traveling UB (PR #214229)

[Yingwei Zheng via llvm-commits]

dtcxzyw wrote:

> I think ideally we'd also say somewhere "UB cannot time-travel except if you put willreturn on a function that has observable behavior", but I am not sure what would be a good place for that.

I think the UB manual has described this behavior: https://llvm.org/docs/UndefinedBehavior.html#time-travel


https://github.com/llvm/llvm-project/pull/214229