Add not enforced check not compatible with mysql 8.0

[jiyfhust]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

CREATE TABLE `t` (`a` int(11) DEFAULT NULL);
insert t values(1);
alter table t ADD CONSTRAINT chk CHECK (a > 1) NOT ENFORCED;

2. What did you expect to see? (Required)

MySQL 8.0:

mysql> alter table t ADD CONSTRAINT chk CHECK (a > 1) NOT ENFORCED;
Query OK, 0 rows affected (0.02 sec)
Records: 0  Duplicates: 0  Warnings: 0

3. What did you see instead (Required)

mysql> alter table t ADD CONSTRAINT chk CHECK (a > 1) NOT ENFORCED;
ERROR 3819 (HY000): Check constraint 'chk' is violated.

4. What is your TiDB version? (Required)

master