excutor: fix the date precision of builtinCastDurationAsStringSig.vecEvalString (#23332)

[wzru]

Signed-off-by: ti-srebot ti-srebot@pingcap.com

cherry-pick #23332 to release-5.0

What problem does this PR solve?

Issue Number: close #26790

Problem Summary:

use test;
drop table if exists t;
create table t(a time(2));
insert into t values("2020-02-02 22:22:22.94");
select length(a),a from t;

release-5.1 & master & mysql 8.0.21(EXPECTED):

mysql> select length(a),a from t;
+-----------+-------------+
| length(a) | a           |
+-----------+-------------+
|        11 | 22:22:22.94 |
+-----------+-------------+
1 row in set (0.00 sec)

release-4.0 & release-5.0:

tidb> select length(a),a from t;
+-----------+-------------+
| length(a) | a           |
+-----------+-------------+
|         8 | 22:22:22.94 |
+-----------+-------------+
1 row in set (0.00 sec)

Release note

None

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• XuHuaiyu
• guo-shaoge

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[ti-chi-bot]

@wzru: This cherry pick PR is for a release branch and has not yet been approved by release team.
Adding the do-not-merge/cherry-pick-not-approved label.

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[wzru]

/cc @XuHuaiyu @guo-shaoge

[XuHuaiyu]

LGTM

[XuHuaiyu]

/merge

[ti-chi-bot]

This pull request has been accepted and is ready to merge.

Commit hash: 46dff7c