[flang] Add optional tdesc operand to fir.embox for polymorphic entities

In some cases, it is useful to be able to embox a polymorphic entity
together with its dynamic type. This patch adds an optional tdesc operand
to fir.embox so the dynamic type can be provided while emboxing the entity.
This will be used in follow-up patch that lowers tbp calls for element of
polymorphic entities array.

Reviewed By: jeanPerier

Differential Revision: https://reviews.llvm.org/D135809