planner: fix a bug that point get plan returns wrong column name (#23365)

[ti-srebot]

cherry-pick #23365 to release-4.0
You can switch your code base to this Pull Request by using git-extras:

# In tidb repo:
git pr https://github.com/pingcap/tidb/pull/23970

After apply modifications, you can push your change to this PR via:

git push git@github.com:ti-srebot/tidb.git pr/23970:release-4.0-fe1cae5f1664

---

What problem does this PR solve?

Issue Number: close #23291

as #23291 (comment) says, the results of TiDB are problematic
also, we explicitly select the column name and expect to get the result with the same column name.

Check List

Tests

• Manual test (add detailed scripts or steps below)

create table t1(`COL1` varchar(255), PRIMARY KEY (`COL1`));
insert into t1 values("a");
SELECT col1 FROM t1 where col1="a";
+------+
| col1 |    -- it should be lower case
+------+
| a    |
+------+
1 row in set

Release note

• fix a bug that point get plan returns wrong column name

[ti-srebot]

/run-all-tests

[ti-srebot]

@xiongjiwei you're already a collaborator in bot's repo.

[djshow832]

/lgtm

[eurekaka]

/lgtm

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• djshow832
• eurekaka

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[xiongjiwei]

/merge

[ti-chi-bot]

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

Commit hash: a226f41

[zhouqiang-cl]

/merge