[Sema] Correct IUnknown to support Unknwnbase.h Header.

Apparently, the MSVC SDK has a strange implementation that
causes a number of implicit functions as well as a template member
function of the IUnknown type. This patch allows these as InterfaceLike
types as well.

Additionally, it corrects the behavior where extern-C++ wrapped around an
Interface-Like type would permit an interface-like type to exist in a namespace.

Differential Revision: https://reviews.llvm.org/D38303

llvm-svn: 314557