[llvm] Add the 'initializes' attribute langref and support (PR #84803)

[Nikita Popov via llvm-commits]

================
@@ -1621,6 +1621,26 @@ Currently, only the following parameter attributes are defined:
     ``readonly`` or a ``memory`` attribute that does not contain
     ``argmem: write``.
 
+``initializes((Lo1, Hi1), ...)``
+    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.
+
+    This attribute only holds for the memory accessed via this pointer
+    parameter. Other arbitrary accesses to the same memory via other pointers
+    are allowed.
+
+    The ``writable`` or ``dereferenceable`` attribute does not imply the
+    ``initializes`` attribute. The ``initializes`` does not imply ``writeonly``
----------------
nikic wrote:

```suggestion
    ``initializes`` attribute. The ``initializes`` attribute does not imply ``writeonly``
```

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