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

[Noam Cohen via cfe-commits]

noamcohen97 wrote:

Thanks @shafik @serge-sans-paille for the review! Updated per your feedback.
Let me know if there's anything else.

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