Use 'location_hash' for 'gcc/diagnostic-spec.h:nowarn_map'

Use 'location_hash' for 'gcc/diagnostic-spec.h:nowarn_map'

Instead of hard-coded '0'/'UINT_MAX', we now use the 'RESERVED_LOCATION_P'
values 'UNKNOWN_LOCATION'/'BUILTINS_LOCATION' as spare values for
'Empty'/'Deleted', and generally simplify the code.

	gcc/
	* diagnostic-spec.h (typedef xint_hash_t)
	(typedef xint_hash_map_t): Replace with...
	(typedef nowarn_map_t): ... this.
	(nowarn_map): Adjust.
	* diagnostic-spec.c (nowarn_map, suppress_warning_at): Likewise.