[flang][hlfir] Implemented addressing an element of a polymorphic array.

The changes convert hlfir.designate to fir.array_coor/fir.embox
to represent a subscripted element of a polymorphic array.
The type information is conveyed via the fir.embox's source_box.

Reviewed By: tblah

Differential Revision: https://reviews.llvm.org/D152200