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

[Arthur Eubanks 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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aeubanks wrote:

We should start this off with a one sentence overview of the attribute. So starting with something like "This indicates that the function initializes the ranges of the pointer parameter's memory." Then describe what "initialize" means. Then the random details like non-overlapping/continuous ranges at the end.

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