[flang] Set declared type when NULLIFY a polymorphic pointer

Fortran standard 7.3.2.3 point 7 mentions that a diassociated
pointer dynamic type is its declared type.
in 9.7.2 note 1, when a NULLIFY statement is applied to a polymorphic pointer,
its dynamic type becomes the same as its declared type.
This patch enforce these standard points by calling the runtime function
`PointerNullifyDerived` with the declared type descriptor.

Reviewed By: jeanPerier

Differential Revision: https://reviews.llvm.org/D136948