[flang] lower vector subscripted polymorphic designators (#84778)

A mold argument need to be added to the hlfir.element_addr and set in
lowering so that when the hlfir.element_addr need to be turned into an
hlfir.elemental operation because the designator must be turned into a
value, the mold can be set on the hlfir.elemental to later allocate the
temporary according the the dynamic type.

This situation happens whenever the vector subscripted polymorphic
designator does not appear as an assignment left-hand side, or as an
IO-input item.


I initially thought retrieving the mold would be tricky if the dynamic
type of the designator was set by a part-ref of the right of the vector
subscripts ("array(vector)%polymorphic_comp"), but this turned out to be
impossible because:
1. A derived type component can be polymorphic only if it has the
POINTER or ALLOCATABLE attribute (F2023 C708).
2. Vector-subscripted part are ranked and F2023 C919 prohibits any
part-ref on the right of the rank part t...