[C++] breakpoint.c: "no memory" software watchpoints and enum casts

Fixes:

 src/gdb/breakpoint.c: In function ‘void update_watchpoint(watchpoint*, int)’:
 src/gdb/breakpoint.c:2147:31: error: invalid conversion from ‘int’ to ‘target_hw_bp_type’ [-fpermissive]
     base->loc->watchpoint_type = -1;
				^

Seems better to rely on "address == -1 && length == -1" than on a enum
value that's not really part of the set of supposedly valid enum
values.  Also, factor that out to separate functions for better
localization of the concept.

gdb/ChangeLog:
2015-11-19  Pedro Alves  <palves@redhat.com>

	* breakpoint.c (software_watchpoint_add_no_memory_location)
	(is_no_memory_software_watchpoint): New functions.
	(update_watchpoint): Use
	software_watchpoint_add_memoryless_location.
	(breakpoint_address_bits): Use is_no_memory_software_watchpoint.