ensure with poly entities are also reconstituted for GC'ed AC

ensure with poly entities are also reconstituted for GC'ed AC

Fixed regression in ORM regarding an internal reconstitution step for the
func:`_orm.with_polymorphic` construct, when the user-facing object is
garbage collected as the query is processed. The reconstitution was not
ensuring the sub-entities for the "polymorphic" case were handled, leading
to an ``AttributeError``.

Fixes: #6680
Change-Id: Id35b16d0f2aadb50b5a7385a21fa81b9d8a8325f