[flang] Accept polymorphic component element in storage_size

When an element is extracted from a polymorphic array, it is
represented as a PolymorphicValue. The PolymorphicValue is
not a boxed value but holds the original polyrmophic array
and the element itself. This was raising an error in storage_size
lowering since we expect a boxed value to take advantage of
fir.box_elesize.
This patch handles PolymorphicValue correctly.

Reviewed By: PeteSteinfeld

Differential Revision: https://reviews.llvm.org/D144643