executor: make group_concat function consider the collation (#27490)

[ti-srebot]

cherry-pick #27490 to release-5.2
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# In tidb repo:
git pr https://github.com/pingcap/tidb/pull/27530

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git push git@github.com:ti-srebot/tidb.git pr/27530:release-5.2-d9bf3bf2d0e0

---

What problem does this PR solve?

Issue Number: close #27429

make group_concat function consider the collation

Check List

Tests

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

Side effects

• Performance regression: Consumes more CPU
• Performance regression: Consumes more Memory
• Breaking backward compatibility

Documentation

• Affects user behaviors
• Contains syntax changes
• Contains variable changes
• Contains experimental features
• Changes MySQL compatibility

Release note

make `group_concat` function consider the collation

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• ichn-hu
• wjhuang2016

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[ti-srebot]

/run-all-tests

[ti-srebot]

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

[zimulala]

@zhouqiang-cl

[zhouqiang-cl]

/merge

[ti-chi-bot]

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

Commit hash: b4d8965

[ti-chi-bot]

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