UPDATE IGNORE doesn't omit the foreign key error

[YangKeao]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

drop table if exists parent, child;
create table parent (id int primary key, ref int, key(ref));
create table child (id int primary key, ref int, foreign key (ref) references parent(ref));
insert into parent values (1, 1), (2, 2);
insert into child values (1, 1);

update child set ref = 2 where id = 1;
-- error 1452
update child set ref = 3 where id = 1;
--enable_warnings
update ignore child set ref = 3 where id = 1;
--disable_warnings

-- error 1451
update parent set ref = 3 where id = 2;
--enable_warnings
update ignore parent set ref = 3 where id = 2;
--disable_warnings

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

TiDB doesn't return error for UPDATE IGNORE statement.

3. What did you see instead (Required)

TiDB returns error.