[flang] Use input type when emboxing/reboxing polymorphic entities

When emboxing an entity to a polymorphic box, use the input type to
compute the type code and element size as the box type is too generic.
When reboxing a polymorphic box, get this information from the input
box.

Reviewed By: jeanPerier

Differential Revision: https://reviews.llvm.org/D139916