gdb: make discrete_position return optional

gdb: make discrete_position return optional

Instead of returning a boolean status and returning the value through a
pointer, return an optional that does both jobs.  This helps in the
following patches, and I think it is an improvement in general.

gdb/ChangeLog:

	* ada-lang.c (ada_value_slice_from_ptr): Adjust.
	(ada_value_slice): Adjust.
	(pos_atr): Adjust.
	* gdbtypes.c (get_discrete_bounds): Adjust.
	(discrete_position): Return optional.
	* gdbtypes.h (discrete_position): Return optional.

Change-Id: I758dbd8858b296ee472ed39ec35db1dbd624a5ae