Allow IUnknown/IInterface types to come from extern C++

It was brought up in response to my last implementation for
this struct-as-interface features that at least 1 header in
the MS SDK uses "extern C++" around an IUnknown declaration.

The previous implementation demanded that this type exist
in the TranslationUnit DeclContext.  This small change simply
also allows in the situation where we're extern "C++".

llvm-svn: 314235