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

[Yingwei Zheng 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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dtcxzyw wrote:

It is mentioned in gep:

> Otherwise, the index itself is sign-extended or truncated, and then multiplied by the type allocation size (that is, the size rounded up to the ABI alignment) of the currently indexed type.

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