fortran: array descriptor: Rename internal field accessor for span [PR122521]

Make the internal accessor for span respect the convention that static
functions don't have the `gfc_' prefix.

	PR fortran/122521

gcc/fortran/ChangeLog:

	* trans-descriptor.cc (gfc_conv_descriptor_span): Rename ...
	(conv_descriptor_span): ... to this.
	(gfc_conv_descriptor_span_get, gfc_conv_descriptor_span_set): Update
	callers.