*: fix the problem that PointGet returns wrong results in the case of overflow (#14776)

[sre-bot]

cherry-pick #14776 to release-4.0

---

What problem does this PR solve?

tidb> create table t6 (a bigint);
Query OK, 0 rows affected (0.00 sec)
tidb> insert into t6 values(9223372036854775807);
Query OK, 1 row affected (0.00 sec)
tidb> alter table t6 add unique key idx(a);
Query OK, 0 rows affected (0.04 sec)

Before this PR:

tidb> select * from t6 where a = 9223372036854775808;
+---------------------+
| a                   |
+---------------------+
| 9223372036854775807 |
+---------------------+
1 row in set, 1 warning (0.01 sec)

After this PR:

tidb>  select * from t6 where a = 9223372036854775808;
Empty set (0.00 sec)

What is changed and how it works?

Do ConvertTo to the point-get value. Handling the overflow and truncated errors.

Check List

Tests

• Unit test

Related changes

• Need to cherry-pick to the release branch

Release note

• Write release note for bug-fix or new feature.

[sre-bot]

/run-all-tests

[zz-jason]

LGTM

[SunRunAway]

LGTM

[sre-bot]

Your auto merge job has been accepted, waiting for:

• 16675

[sre-bot]

/run-all-tests

[sre-bot]

@sre-bot merge failed.

[zz-jason]

/run-check_dev