Clarify 'key_type_t' to 'location_t' as used for 'gcc/diagnostic-spec.h:nowarn_map'

Clarify 'key_type_t' to 'location_t' as used for 'gcc/diagnostic-spec.h:nowarn_map'

To make it obvious what exactly the key type is.  No change in behavior.

	gcc/
	* diagnostic-spec.h (typedef xint_hash_t): Use 'location_t' instead of...
	(typedef key_type_t): ... this.  Remove.
	(nowarn_map): Document.
	* diagnostic-spec.c (nowarn_map): Likewise.
	* warning-control.cc (convert_to_key): Evolve functions into...
	(get_location): ... these.  Adjust all users.