gdb: make get_discrete_bounds return bool

gdb: make get_discrete_bounds return bool

get_discrete_bounds currently has three possible return values (see its
current doc for details).  It appears that for all callers, it would be
sufficient to have a boolean "worked" / "didn't work" return value.

Change the return type of get_discrete_bounds to bool and adjust all
callers.  Doing so simplifies the following patch.

gdb/ChangeLog:

	* gdbtypes.h (get_discrete_bounds): Return bool, adjust all
	callers.
	* gdbtypes.c (get_discrete_bounds): Return bool.

Change-Id: Ie51feee23c75f0cd7939742604282d745db59172