[flang] Do not query type_desc for unlimited polymoprhic entities in move_alloc

In D144997, the dynamic type of polymorphic entities is reset to the declared
type when the FROM is deallocated. To do this, the declared type was passed as
a fir.type_desc op. For unlimited polymorphic entities, this should just be a
null pointer.

Reviewed By: PeteSteinfeld

Differential Revision: https://reviews.llvm.org/D145380