ddl: fix duplicate elementID allocation to make sure gc work for partition table (#33726)

[ti-srebot]

cherry-pick #33726 to release-5.2
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# In tidb repo:
git pr https://github.com/pingcap/tidb/pull/33854

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---

What problem does this PR solve?

Issue Number: close #33620

Problem Summary:
We use partitionID or indexID as elementID sometimes.
When drop index for partition table, we use indexID as elementID. So we may miss some deleteRange.

What is changed and how it works?

Allocate elementID as 1, 2, 3, 4 for a DDL job.
Besides, add a comment to log so that we can debug easier.

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 duplicate elementID allocation to make sure gc work for partition table

[ti-chi-bot]

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[ti-srebot]

/run-all-tests

[ti-srebot]

@wjhuang2016 you're already a collaborator in bot's repo.

[VelocityLight]

This pull request is closed because it's related version has closed automatic cherry-picking.
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