[clang] Add support for anyAppleOS availability (PR #181953)

[Allan Shortlidge via cfe-commits]

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@@ -2697,6 +2698,70 @@ static void handleAvailabilityAttr(Sema &S, Decl *D, const ParsedAttr &AL) {
     }
   }
 
+  // Handle anyAppleOS specially: create implicit platform-specific attributes
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tshortli wrote:

I don't think this should block this PR from landing as it would be really helpful to get the basic support in sooner rather than later. Maybe we can come up with a follow up that preserves the original attribute from source in the AST, even if clang also expands it implicitly for its own purposes.

https://github.com/llvm/llvm-project/pull/181953