[flang] Handle pointer assignment with polymorphic entities

This patch forces pointer and allocatable polymorphic entities to be
tracked as descriptor. It also enables the pointer assignment between
polymorphic entities. Pointer association between a non-polymorphic
pointer and a polyrmophic target might require some more work as
per 10.2.2.3 point 1.

Reviewed By: PeteSteinfeld

Differential Revision: https://reviews.llvm.org/D137150