[flang] implement sizeof lowering for polymorphic entities (#84498)

For non polymorphic entities, semantics knows the type size and rewrite
sizeof to `"cst element size" * size(x)`.

Lowering has to deal with the polymorphic case where the type size must
be retrieved from the descriptor (note that the lowering implementation
would work with any entity, polymorphic on not, it is just not used for
the non polymorphic cases).