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

[Jan Voung via llvm-commits]

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@@ -1621,6 +1621,28 @@ Currently, only the following parameter attributes are defined:
     ``readonly`` or a ``memory`` attribute that does not contain
     ``argmem: write``.
 
+``initialized((Lo1,Hi1),...)``
+    This attribute is a list of const ranges in ascending order with no
+    overlapping or continuous. It indicates that the function initializes the
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jvoung wrote:

nit: "with no overlapping or adjoining list elements"? or something like that (felt like it was missing a word at the end at least)

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