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

[via llvm-commits]

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@@ -105,6 +106,9 @@ optimize the function to simply:
       unreachable
     }
 
+Without ``willreturn``, UB in LLVM will never exhibit time-traveling. This means
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gonzalobg wrote:

@RalfJung are we sure that ``willreturn`` is the only mechanism available to trigger time-traveling?

I think having a list of all features that can lead to time-traveling would be great, and starting that list by documenting that `willreturn` is one such feature is a great start.

But we have many features, and I don't know if this is the only one that can lead to this.
++ @jyknight for review since I don't know (e.g. an indirect call of a convergence control intrinsic is undefined behavior, who knows..).

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