[flang] Restore declared type when deallocating polymorphic entities

[flang] Restore declared type when deallocating polymorphic entities

As mentioned in section 7.3.2.3 note 7, The dynamic type of an unallocated
allocatable object or a disassociated pointer is the same as its declared type.

This patch adds two function to the runtime:
- `PointerDeallocatePolymorphic`
- `AllocatableDeallocatePolymorphic`

These two functions take a DerivedTypeDesc pointer of the declared type.
The lowering is updated accordingly to call these functions for polymorphic
and unlimited polyrmophic entities. For unlimited polymorphic entities, the
dynamic type is set to nullptr when the entity is on an unallocated or
disassociated state.

Reviewed By: PeteSteinfeld, klausler

Differential Revision: https://reviews.llvm.org/D141519