[llvm] Add 'initialized' attribute langref (PR #84803)

[Arthur Eubanks via llvm-commits]

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@@ -1621,6 +1621,31 @@ Currently, only the following parameter attributes are defined:
     ``readonly`` or a ``memory`` attribute that does not contain
     ``argmem: write``.
 
+``initialized((Lo1,Hi1),...)``
+    This attribute indicates that the function initializes the ranges of the
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aeubanks wrote:

`This attribute indicates that the function initializes the ranges of the pointer parameter's memory ``[%p+LoN, %p+HiN)``. Initialization of memory means the first memory access is a non-volatile, non-atomic write. The write must happen before the function returns. If the function unwinds, the write may not happen.`

we might be able to relax the non-atomic part later?

https://github.com/llvm/llvm-project/pull/84803