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

[Vlad Serebrennikov via cfe-commits]

Endilll wrote:

> It'd be very nice if we can get this PR done before 22.1.0-rc2, which is scheduled for 27th

After an offline discussion @DeinAlptraum considered it doable to address the entirety of #156680 in the Clang 23 timeframe, and I agreed that it would be better to address all the issues listed there in a single wave of deprecations (and then removals) to make it less painful for users. So this PR is no longer planned to be backported to 22 branch.

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