[llvm/Object] - Make dyn_cast<XCOFFObjectFile> work as it should.

Currently, `dyn_cast<XCOFFObjectFile>` always does cast and returns a pointer,
even when we pass `ELF`/`Wasm`/`Mach-O` or `COFF` instead of `XCOFF`.

It happens because `XCOFFObjectFile` class does not implement `classof`.
I've fixed it and added a unit test.

Differential revision: https://reviews.llvm.org/D86542