Fix opclass/opfamily identity strings

The original representation uses "opcname for amname", which is good
enough; but if we replace "for" with "using", we can apply the returned
identity directly in a DROP command, as in

DROP OPERATOR CLASS opcname USING amname

This slightly simplifies code using object identities to programatically
execute commands on these kinds of objects.

Note backwards-incompatible change:
The previous representation dates back to 9.3 when object identities
were introduced by commit f8348ea3, but we don't want to change the
behavior on released branches unnecessarily and so this is not
backpatched.

src/backend/catalog/objectaddress.c

@@ -3533,7 +3533,7 @@ getObjectIdentityParts(const ObjectAddress *object,
 				appendStringInfoString(&buffer,
 									   quote_qualified_identifier(schema,
 												 NameStr(opcForm->opcname)));
-				appendStringInfo(&buffer, " for %s",
+				appendStringInfo(&buffer, " USING %s",
 								 quote_identifier(NameStr(amForm->amname)));
 				if (objname)
 				{
@@ -4070,7 +4070,7 @@ getOpFamilyIdentity(StringInfo buffer, Oid opfid, List **objname, List **objargs
 	amForm = (Form_pg_am) GETSTRUCT(amTup);
 
 	schema = get_namespace_name(opfForm->opfnamespace);
-	appendStringInfo(buffer, "%s for %s",
+	appendStringInfo(buffer, "%s USING %s",
 					 quote_qualified_identifier(schema,
 												NameStr(opfForm->opfname)),
 					 NameStr(amForm->amname));

src/test/regress/expected/object_address.out

@@ -373,11 +373,11 @@ SELECT (pg_identify_object(addr1.classid, addr1.objid, addr1.subobjid)).*,
  conversion                | pg_catalog | ascii_to_mic      | pg_catalog.ascii_to_mic                                              | t
  language                  |            | plpgsql           | plpgsql                                                              | t
  schema                    |            | addr_nsp          | addr_nsp                                                             | t
- operator class            | pg_catalog | int4_ops          | pg_catalog.int4_ops for btree                                        | t
+ operator class            | pg_catalog | int4_ops          | pg_catalog.int4_ops USING btree                                      | t
  operator                  | pg_catalog |                   | pg_catalog.+(integer,integer)                                        | t
  rule                      |            |                   | "_RETURN" on addr_nsp.genview                                        | t
  trigger                   |            |                   | t on addr_nsp.gentable                                               | t
- operator family           | pg_catalog | integer_ops       | pg_catalog.integer_ops for btree                                     | t
+ operator family           | pg_catalog | integer_ops       | pg_catalog.integer_ops USING btree                                   | t
  policy                    |            |                   | genpol on addr_nsp.gentable                                          | t
  collation                 | pg_catalog | "default"         | pg_catalog."default"                                                 | t
  text search dictionary    | addr_nsp   | addr_ts_dict      | addr_nsp.addr_ts_dict                                                | t