[flang] Handle non derived-type unlimited polymorphic allocation

Runtime call to PointerNullifyDerived or AllocatableInitDerived
should only be generated for derived-type allocation of polymorphic entities.
With unlimited polymorphic entities, it is possible that the type spec is not
a derived-type. Avoid failure in that case.

Reviewed By: PeteSteinfeld

Differential Revision: https://reviews.llvm.org/D137353