mscoree: Implement DllGetClassObject

[Vincent Povirk]

> I found if you used GetIUnknownForObject and then queried for the IDispatch
> interface, mono returned E_NOINTERFACE. Maybe this is a bug with mono.

If that's true, using IDispatch instead of IUnknown is probably not an
acceptable work-around, as we don't know how that works when a class
explicitly implements a non-IDispatch interface that the caller
requests. It sounds like a mono bug to me, but it needs a testcase.