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

[yu34po]

What problem does this PR solve?

What is changed and how it works?

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)
• No code

Code changes

• Has exported function/method change
• Has exported variable/fields change
• Has interface methods change
• Has persistent data change

Side effects

• Possible performance regression
• Increased code complexity
• Breaking backward compatibility

Related changes

• Need to cherry-pick to the release branch
• Need to update the documentation
• Need to update the tidb-ansible repository
• Need to be included in the release note

[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.

[shenli]

@yu34po Please merge this PR to the master first and refine the description.

[zimulala]

LGTM

[zz-jason]

LGTM

[zz-jason]

/run-all-tests tidb-test=release-2.1 tikv=release-2.1 pd=release-2.1

[zz-jason]

@yu34po please cherrypick this commit to release-2.0 as well. 😄