[flang] Lower and code gen for allocate on polymorphic entities

When allocating a polymorphic entity, its type descriptor can come
from the declared type or can be provided in the allocate statement.
This patch adds lowering for allocate on polymorphic by calling
the `AllocatableInitDerived` runtime function with the correct
type descriptor. Some adaptation are made in the code generation
to accept fir.class where it is appropriate.

Reviewed By: jeanPerier

Differential Revision: https://reviews.llvm.org/D136426