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

[Chuanqi Xu via cfe-commits]

https://github.com/ChuanqiXu9 commented:

Patch itself looks not bad at the first glance. But due to I don't use ObjC. Let me add apple people for apple things 

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