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

[Vlad Serebrennikov via cfe-commits]

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Endilll wrote:

I think we should have three entries in release notes:
1. Removal of `CompletionString.Availability` (no changes needed here).
2. Change of the return type of `CompletionString.availability`. This is where all the text about `AvailabilityKindCompat` goes
3. Removal of `availabilityKinds` as a consequence of the previous entry. This is where the last sentence about migration from `availabilityKinds` should be attached to.

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