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

[Jannick Kremer via cfe-commits]

DeinAlptraum wrote:

> Is there anything preventing us from providing our own `__str__` for `AvailabilityKind`?

There are currently two ways to get AvailabilityKinds out of the API (except for intializing them yourself): accessing the `availability` property on a `Cursor`, or the property of the same name on a `CompletionString`.

Changing the `__str__` of `AvailabilityKind` directly would mean a breaking change over there instead.

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