[flang] Use tdesc on fir.embox in code generation

For polymoprhic entities, the type descriptor is dynamic. The tdesc
operand on fir.embox is meant to propagate the dynamic type when
embox a polymorphic entity.
This patch makes use of this operand in code generation and update
the created descriptor accordingly.

Reviewed By: jeanPerier

Differential Revision: https://reviews.llvm.org/D136748