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

[Ralf Jung via llvm-commits]

RalfJung 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

Ah nice! I extended that section.

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