[clang] [ObjC] Enable diagnose_if on Objective-C methods (PR #115056)

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apple-fcloutier wrote:

An abstraction that includes blocks doesn't help us here because we are looking for the combination of `NamedDecl` and `DeclContext` and `BlockDecl` isn't a `NamedDecl`.

Aaron, I'm not sure I understand you correctly, but ObjCMethodDecl doesn't have a type. What would it mean to tie BlockDecl, FunctionDecl and ObjCMethoDecl at the type level?

https://github.com/llvm/llvm-project/pull/115056