[clang] [CIR] Implement vTableClassName for Type::ObjCObject (PR #195232)

[Dylan Leifer-Ives via cfe-commits]

dleiferives wrote:

> I'm honestly a bit surprised that the test case here compiles. I don't think we've really done anything to support Objective-C. Is there a reason that you're implementing this, beyond just wanting to contribute to the open GitHub issue?

I've definitely learned that the Objective-C support is sparse. Finding a test case took a lot of digging. `typeid()` with the builtin Objective-C types was the only way that I could find that avoids `convertType(ObjCObjectPointerType)`, which is NYI a few layers deep everywhere else. 

I want to learn ClangIR internals as it's required for a research project I'm working on, so I picked what seemed like a doable beginner issue to dig into (learn by doing and all that). So no, nothing beyond the open issue.

https://github.com/llvm/llvm-project/pull/195232