Change breakpoints to use value_ref_ptr

Change breakpoints to use value_ref_ptr

Now that value_ref_ptr exists, it is possible to simplify breakpoint
and bpstat memory management by using a value_ref_ptr rather than
manually handling the reference counts.

gdb/ChangeLog
2018-04-06  Tom Tromey  <tom@tromey.com>

	* value.c (release_value): Update.
	* breakpoint.h (struct watchpoint) <val>: Now a value_ref_ptr.
	(struct bpstats) <val>: Now a value_ref_ptr.
	* breakpoint.c (update_watchpoint, breakpoint_init_inferior)
	(~bpstats, bpstats, bpstat_clear_actions, watchpoint_check)
	(~watchpoint, print_it_watchpoint, watch_command_1)
	(invalidate_bp_value_on_memory_change): Update.