fortran: array descriptor: Move null factory function [PR122521]

Move null descriptor factory function to trans-descriptor.cc.

	PR fortran/122521

gcc/fortran/ChangeLog:

	* trans-array.cc (DATA_FIELD, OFFSET_FIELD, DTYPE_FIELD, SPAN_FIELD,
	DIMENSION_FIELD, CAF_TOKEN_FIELD, STRIDE_SUBFIELD, LBOUND_SUBFIELD,
	UBOUND_SUBFIELD): Remove preprocessor constants.
	(gfc_build_null_descriptor): Move function to ...
	* trans-descriptor.cc (gfc_build_null_descriptor): ... this file.
	* trans-array.h (gfc_build_null_descriptor): Move declaration to ...
	* trans-descriptor.h (gfc_build_null_descriptor): ... this file.