Change ObjCInterfaceDecl to inherit from NamedDecl (not TypeDecl). While...

Change ObjCInterfaceDecl to inherit from NamedDecl (not TypeDecl). While ObjCInterfaceDecl is arguably a TypeDecl, it isn't a ScopedDecl. Since TypeDecl's are scoped, it makes sense to simply treat them as NamedDecl's. I could have fiddled a bit more with the hierarchy (in terms of creating a non-scoped TypeDecl), however this probably isn't worth the effort. 

I also finished unifying access to scope decl change by converting Sema::getObjCInterfaceDecl() to use Sema::LookupDecl(). This is much cleaner now:-)

llvm-svn: 49107