fortran: array descriptor: Move bound shift utility function [PR122521]

Move gfc_conv_shift_descriptor_lbound to trans-descriptor.cc

	PR fortran/122521

gcc/fortran/ChangeLog:

	* trans-array.cc (gfc_conv_shift_descriptor_lbound): Move
	function ...
	* trans-descriptor.cc (gfc_conv_shift_descriptor_lbound): ... to
	this file.
	* trans-array.h (gfc_conv_shift_descriptor_lbound): Move
	declaration ...
	* trans-descriptor.h (gfc_conv_shift_descriptor_lbound): ... to this
	file.