[clang] [clang] ast-dump: dump `AvailabilityAttr` fields to JSON (PR #179281)

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serge-sans-paille wrote:

LGTM. As a higher level note, we should do something to make sure we always have perfect handling for this dumper, but that's out of scope for this PR. 

https://github.com/llvm/llvm-project/pull/179281