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.

sql/sql_class.cc

@@ -5532,6 +5532,7 @@ bool xid_cache_insert(THD *thd, XID_STATE *xid_state)
     break;
   case 1:
     my_error(ER_XAER_DUPID, MYF(0));
+    /* fall through */
   default:
     xid_state->xid_cache_element= 0;
   }