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

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++".

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@314235 91177308-0d34-0410-b5e6-96231b3b80d8