[clang] [Clang] Fix isWeakImported() to traverse redeclaration chain for avai… (PR #181482)

[Victor Chernyakin via cfe-commits]

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@@ -0,0 +1,8 @@
+// Mimics a class like UTType that has availability attrs for multiple platforms.
+// The 'macos' attr comes before 'ios', so getAttr<AvailabilityAttr>() returns
+// 'macos' first. When mergeInheritableAttributes copies only the first attr
+// across PCM boundaries, the 'ios' attr is lost on the @class redeclaration.
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localspook wrote:

Ah, if the problem is that only the first `AvailabilityAttr` is being propagated, I would imagine the solution is to change `mergeInheritableAttributes` to propagate all of them? Instead of adding special behavior in `isWeakImported`. (can someone more familiar with this area weigh in?)

https://github.com/llvm/llvm-project/pull/181482