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

[Ralf Jung 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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RalfJung wrote:

Assuming you are referring to ["convergent operations"](https://llvm.org/docs/ConvergentOperations.html), then as far as I can tell they don't have semantics in the AM, they are a hack that seems to mostly work. I don't think it makes sense to ask detailed questions about them in their current state.

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