[clang] Redesign handling of anyAppleOS availability attribute (PR #190817)

[Cyndy Ishida via cfe-commits]

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cyndyishida wrote:

Could you add a testcase that legitimate conflicting availabilities are still diagnosed? e.g. 

e.g. ` __attribute__((availability(anyAppleOS, introduced=26.0)))  __attribute__((availability(anyAppleOS, introduced=27.0)));  __attribute__((availability(iOS, introduced=28.0)));`

https://github.com/llvm/llvm-project/pull/190817