[flang] Reset dynamic type for optional intent(out) allocatable polymorphic dummy

Allocatable intent(out) are deallocated at the beginning of a function/subroutine.
For polyrmophic entities, the dynamic type need to be reseted to the declared
type. This patch makes sure this is done when the dummy argument is optional and
present.

Depends on D145674

Reviewed By: PeteSteinfeld

Differential Revision: https://reviews.llvm.org/D145679