[llvm] [LangRef] allow omitting `va_end` (PR #203087)

[Eli Friedman via llvm-commits]

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@@ -14671,6 +14677,10 @@ available in C. In a target-dependent way, it copies the source
 intrinsic is necessary because the ``llvm.va_start`` intrinsic may be
 arbitrarily complex and require, for example, memory allocation.
 
+On targets where ``llvm.va_copy`` is equivalent to ``memcpy``, ``memcpy``
+can be used instead to duplicate a ``va_list``. ``llvm.va_copy`` is
+equivalent to ``memcpy`` on all currently supported targets implement
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efriedma-quic wrote:

```suggestion
equivalent to ``memcpy`` on all currently supported targets.
```

https://github.com/llvm/llvm-project/pull/203087