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

[Aaron Ballman via cfe-commits]

AaronBallman wrote:

I agree that having something tying these together would be nice, but should we also tie them together at the `Type` level similarly, or just the `Decl` level?

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