[flang] Ensure "dtv" args passed by descriptor when polymorphic

The predicate used to determine whether the initial "dtv" argument
to a user-defined derived type I/O procedure specified by a generic
interface that is not in the derived type definition is wrong.
The "dtv" argument must be passed via a descriptor whenever it is
polymorphic.

Differential Revision: https://reviews.llvm.org/D145255