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

[via cfe-commits]

apple-fcloutier wrote:

I wish I didn't need to change the ExprConstant call stack machinery like this, happy to take suggestions. This is all because the closest ancestors of FunctionDecl and ObjCMethodDecl are NamedDecl and DeclGroup.

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