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

[Eli Friedman 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
+frontends can avoid time-traveling UB by only putting ``willreturn`` on
+functions without observable behavior.
 
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efriedma-quic wrote:

It's maybe worth clarifying that the lack of "time-travelling" only applies to things which are formally allowed to stop execution (volatile memory acccesses/IO/etc.)  LLVM can and will infer willreturn for functions that have contain atomics and other similar constructs.

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