expression: fix wrong caseWhen function for enum type (#29454)

[ti-srebot]

cherry-pick #29454 to release-5.2
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# In tidb repo:
git pr https://github.com/pingcap/tidb/pull/29511

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git push git@github.com:ti-srebot/tidb.git pr/29511:release-5.2-ae65cae8d04c

---

What problem does this PR solve?

Issue Number: close #29357

Problem Summary:

What is changed and how it works?

There are two problem in this issue:
Problem 1:

1. caseWhenFunctionClass.getFunction get fieldType Enum from children, and construct scalarFunction caseWhenString.
2. compareFunctionClass.getFunction get fieldType Int and Enum, and set EnumSetAsInt flag, but not re-construct caseWhen function.
3. Get the final scalarFunction EQInt(1, CaseWhenString(0, Enum)) and pushdown. CaseWhenString can't get int value.

Problem 2:

[tidb]> SELECT 5 = (case when 0 <=> 1 then a else a end) from t;
ERROR 1105 (HY000): baseBuiltinFunc.vecEvalInt() should never be called, please contact the TiDB team for help

1. BuildCastFunction build a cast CastStringAsString, and the fieldType is Enum.
2. compareFunctionClass.getFunction get fieldType Int and Enum, and set EnumSetAsInt flag, but not re-construct Cast function.
3. Get the final scalarFunction EQInt(1, CastStringAsString(Enum))). CastStringAsString can't get int value.
   BuildCastFunction is useless. See constant_fold.goL129. We should have the same behavior for all args.

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

Fix panic for caseWhen function with enum type

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• XuHuaiyu
• ichn-hu

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[ti-srebot]

/run-all-tests

[ti-srebot]

@wshwsh12 you're already a collaborator in bot's repo.

[XuHuaiyu]

LGTM

[wshwsh12]

/merge

[ti-chi-bot]

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

Commit hash: f9665df

[sre-bot]

Code Coverage Details: https://codecov.io/github/pingcap/tidb/commit/96353a5105605df6a893389ba6b991d7c68c4130

[ti-chi-bot]

@ti-srebot: Your PR was out of date, I have automatically updated it for you.

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[wshwsh12]

/run-unit-test