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

[Jannick Kremer via cfe-commits]

DeinAlptraum wrote:

> Can you also add a test to ensure that nothing breaks?

What would you like to be tested?
We already have tests asserting exact `CompletionString`s, see here: https://github.com/llvm/llvm-project/blob/077a796c0da6c32d5d43af9945b4c2728f43633f/clang/bindings/python/tests/cindex/test_code_completion.py#L51-L55

which I used to ensure that the format is correct (I kept adapting the converter function above until these tests passed)

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