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

[John McCall via cfe-commits]

rjmccall wrote:

Having an abstraction for this would be nice. It doesn't *have* to be a base class specifically, though; we can do a more extrinsic tagged-union without actually unifying the class hierarchies, like `llvm::CallSite` does. Swift has a type called `AnyFunctionRef` that unifies several different kinds of entity that behave like functions.

I feel like most of the uses of this abstraction would want it to cover blocks, though.  Basically a union of every kind of code context.

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