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

[Ralf Jung via llvm-commits]

================
@@ -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.
 
----------------
RalfJung wrote:

Thanks!
I think I have enough green checkmarks then. Going to merge :)

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