fortran: array descriptor: Add internal field accessor for data [PR122521]

Add a static function producing a reference to the data field of array
descriptors, like those existing for the other fields.

	PR fortran/122521

gcc/fortran/ChangeLog:

	* trans-descriptor.cc (conv_descriptor_data): New function.
	(gfc_conv_descriptor_data_get, gfc_conv_descriptor_data_set): Use
	it.