fortran: array descriptor: Move nullification code [PR122521]

Move the pointer nullification implementing code to its own function in
trans-descriptor.cc.

	PR fortran/122521

gcc/fortran/ChangeLog:

	* trans-expr.cc (gfc_trans_pointer_assignment): Move pointer
	nullification generation ...
	* trans-descriptor.cc (gfc_nullify_descriptor): ... here as a new
	function.
	* trans-descriptor.h (gfc_nullify_descriptor): New declaration.