[clang] Use existing AvailabilityKind enum for code completion availability (PR #160296)

[Vlad Serebrennikov via cfe-commits]

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@@ -54,6 +54,19 @@ Clang Frontend Potentially Breaking Changes
 
 Clang Python Bindings Potentially Breaking Changes
 --------------------------------------------------
+- Remove ``CompletionString.Availability``. No libclang interfaces returned instances of it.
+- Remove ``CompletionString.Kind`` and its container, ``availabilityKinds``.
+  ``CompletionString.availability`` now returns instances of ``CompletionString.AvailabilityKindCompat``.
+
+  Instances of ``AvailabilityKindCompat`` have the same ``__str__`` representation
+  as the previous ``CompletionString.Kind``s and are equality-compatible with
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Endilll wrote:

```suggestion
  as the previous ``CompletionString.Kind``s and are equality-comparable with
```

https://github.com/llvm/llvm-project/pull/160296