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

[Jannick Kremer via cfe-commits]

DeinAlptraum wrote:

I think it's also worth asking whether we want to go with this approach at all: this is already a breaking change w.r.t. the string representations of the `availability` attribute of `CompletionString`.
What's our plan with `AvailabilityKind` as it is presented in `CompletionString`'s `__repr__`? Do we want to eventually change this to use the same string representation as all other kind enums? Should it remain like this forever?
In the former case it might make sense to do this breaking changes together, but I'm really not sure what's the best option here anyway

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