[flang] Carry the whole polymorphic box in PolymorphicValue

[flang] Carry the whole polymorphic box in PolymorphicValue

Until now, only the address of the type descriptor was hold in
a PolymorphicValue. In some cases, the element size and the
type code are also needed when creating new polymorphic
descriptors from an element of a polymorphic entity.

This patch updates PolymorphicValue to carry the source
descriptor from which the element is extracted. The source
descriptor is then used when emboxing the element to a new
polymorphic descriptor.

This simplify the code done in D141274 and will be used
when creating polymorphic temporary as well.

Reviewed By: jeanPerier, PeteSteinfeld

Differential Revision: https://reviews.llvm.org/D141609