[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
git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@314557
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