Silence bogus GCC 7 warnings -Wimplicit-fallthrough

Do not silence uncertain cases, or fix any bugs.

The only functional change should be that ha_federated::extra()
is not calling DBUG_PRINT to report an unhandled case for
HA_EXTRA_PREPARE_FOR_DROP.

client/mysqldump.c

@@ -2168,6 +2168,7 @@ static void print_xml_comment(FILE *xml_file, size_t len,
     case '-':
       if (*(comment_string + 1) == '-')         /* Only one hyphen allowed. */
         break;
+      /* fall through */
     default:
       fputc(*comment_string, xml_file);
       break;