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_table.cc

@@ -6851,7 +6851,8 @@ bool alter_table_manage_keys(TABLE *table, int indexes_were_disabled,
   case Alter_info::LEAVE_AS_IS:
     if (!indexes_were_disabled)
       break;
-    /* fall-through: disabled indexes */
+    /* disabled indexes */
+    /* fall through */
   case Alter_info::DISABLE:
     error= table->file->ha_disable_indexes(HA_KEY_SWITCH_NONUNIQ_SAVE);
   }