[llvm] [LangRef] Add a description of the semantics of call signatures. (PR #136189)

[Eli Friedman via llvm-commits]

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@@ -13152,6 +13155,16 @@ values. Upon a '``ret``' instruction in the called function, control
 flow continues with the instruction after the function call, and the
 return value of the function is bound to the result argument.
 
+If the callee refers to an intrinsic function, the signature of the call must
+match the signature of the callee.  Otherwise, if the signature of the call
+does not match the signature of the called function, the behavior is
+target-specific.  For a significant mismatch, this likely results in undefined
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efriedma-quic wrote:

There's some weirdness here because of the way ABI lowering works: we don't define one correct way to lower a high-level type to an LLVM type, so different frontends represent the "same" type as a different LLVM type.  Because of this, optimizations are conservative with signature mismatches.  Maybe we can revisit this at some point.

https://github.com/llvm/llvm-project/pull/136189