expression: refine built-in func truncate to support uint arg (#8000)

[yu34po]

What problem does this PR solve?

fix #7591
fix #7615

What is changed and how it works?

1.add an builtinTruncateUintSig Function to support unsigned int truncate
2.change builtinTruncateIntSig truncate function to avoid get incorrect value

Check List

Tests

• Unit test
• Integration test

Code changes

• Has exported function/method change

Side effects

Related changes

[sre-bot]

Hi contributor, thanks for your PR.

This patch needs to be approved by someone of admins. They should reply with "/ok-to-test" to accept this PR for running test automatically.

[winoros]

@yu34po Please fill the pr description.

[XuHuaiyu]

@yu34po
We need do some modification, since the input argument type if types.Row:
func (b *builtinTruncateIntSig) evalInt(row types.Row) (int64, bool, error) {...}

[lysu]

/run-all-tests tidb-test=release-2.0 tikv=release-2.0 pd=release-2.0

[WangXiangUSTC]

/run-all-tests tidb-test=release-2.0 tikv=release-2.0 pd=release-2.0

[zz-jason]

LGTM
@XuHuaiyu PTAL

[XuHuaiyu]

LGTM