[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
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+- Remove ``CompletionString.Availability``. No libclang interfaces returned instances of it.
+- Remove ``CompletionString.Kind`` and its container, ``availabilityKinds``.
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Endilll wrote:

We couldn't remove `CompletionString.Kind`, because it doesn't exist. We couldn't remove `CompletionChunk.Kind` either, because it's used elsewhere

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