Fix the __interface inheritence rules to work better with IUnknown and IDispatch

__interface objects in MSVC are permitted to inherit from __interface types, 
and interface-like types.

Additionally, there are two default interface-like types 
(IUnknown and IDispatch) that all interface-like
types must inherit from.

Differential Revision: https://reviews.llvm.org/D37308

llvm-svn: 313364