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

[Akira Hatanaka via cfe-commits]

ahatanak 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.

Should we enable `diagnose_if` for block methods too?

```
void (^block)(int a) __attribute__((diagnose_if(a < 0, "cannot pass negative numbers", "error")));
```

Also is it possible or better to make `CallableDecl` an abstract base class of `FunctionDecl`, `ObjCMethodDecl` and `BlockDecl`?

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