Cast int as time may produce wront result

[leiysky]

Minimal reproduce:

create table t (a int);
insert into t values(65535);
alter table t set tiflash replica 1;
set session tidb_isolation_read_engines='tiflash';
select * from t where cast(a as datetime);
# expect empty but get 65535

[leiysky]

The root cause of this bug is that TiFlash doesn't validate the result of cast integer as datetime as strict as TiDB.

This inconsistent behavior only occurs when the original integer value has valid format but doesn't represent a valid datetime value. For example a integer 20210499 can be casted to datetime '2021-04-99', in TiDB it will return NULL while in TiFlash it will return '2021-04-99'.

We can change the severity to major since this is a corner case.