[flang] Carry dynamic type when emboxing polymorphic pointer

In order to be passed as passed-object in the dynamic dispatch, the
polymorphic pointer entity are emboxed. In this process, the dynamic
type must be preserve and pass to fir.embox as the tdesc operand. This
patch introduce a new ExtendedValue that allow to carry over the
dynamic type when the value is unboxed.

Depends on D136820

Reviewed By: PeteSteinfeld

Differential Revision: https://reviews.llvm.org/D136824