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

[Vlad Serebrennikov via cfe-commits]

================
@@ -54,6 +54,17 @@ Clang Frontend Potentially Breaking Changes
 
 Clang Python Bindings Potentially Breaking Changes
 --------------------------------------------------
+- Remove ``CompletionString.Availability``. No libclang interfaces returned instances of it.
+- ``CompletionString.availability`` now returns instances of ``CompletionString.AvailabilityKindCompat``.
+
+  Instances of ``AvailabilityKindCompat`` have the same ``__str__`` representation
+  as the previous ``CompletionChunk.Kind``s and are equality-comparable with
+  the existing ``AvailabilityKind`` enum. It will be replaced by ``AvailabilityKind``
+  in a future release. When this happens, the return type of ``CompletionString.availability``
+  will change to ``AvailabilityKind``, so it is recommended to use ``AvailabilityKind``
+  to compare with the return values of ``CompletionString.availability``.
+- Remove ``availabilityKinds``. In this release, uses of ``availabilityKinds``
+  need to be replaced by ``CompletionString.AvailabilityKindCompat``.
----------------
Endilll wrote:

```suggestion
  need to be replaced by ``CompletionString.AvailabilityKind``.
```

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