Do not register ObjCInterfaceDecls implicitly created by @class in the

current DeclContext.  These "imaginary" declarations pose issues for
clients searching DeclContext for actual declarations.  Instead,
register them for name lookup, and add the ObjCInterfaceDecl later to
the DeclContext when we hit an actual @interface declaration.

This also fixes a bug where the invariant that the Decls in a
DeclContext are sorted in order of their appearance is no longer
violated.  What could happen is that an @class causes an
ObjCInterfaceDecl to get added first to the DeclContext, then the
ObjCClassDecl itself is added, and then later the SourceLocation of
the ObjCInterfaceDecl is updated with the correct location (which is
later in the file).  This breaks an assumed invariant in
ResolveLocation.cpp (and possibly other clients).

llvm-svn: 89160