[llvm] [LangRef] Clarifying the copying behaviour of byval (PR #205576)

[Ralf Jung via llvm-commits]

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@@ -1357,7 +1357,11 @@ Currently, only the following parameter attributes are defined:
     ``byval`` parameters). This is not a valid attribute for return
     values.
 
-    The byval type argument indicates the in-memory value type.
+    The byval type argument is only used for its allocation size and alignment
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RalfJung wrote:

defining "en passant"... would be nice to put this more properly somewhere in the types section. But that's not a blocker for this PR.

https://github.com/llvm/llvm-project/pull/205576