ddl: make prefix index compatible with mysql 8.0 when prefix length equal to column length | tidb-test=pr/2308 (#48296)

[ti-chi-bot]

This is an automated cherry-pick of #48296

What problem does this PR solve?

Issue Number: close #48295

Problem Summary:

What is changed and how it works?

Check List

Tests

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

  • I checked and no code files have been changed.

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

Please refer to Release Notes Language Style Guide to write a quality release note.

None

[ti-chi-bot]

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

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[ti-chi-bot]

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[ti-chi-bot]

@ti-chi-bot: The following tests failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
idc-jenkins-ci-tidb/mysql-test	eee3a83	link	true	/test mysql-test
idc-jenkins-ci-tidb/check_dev_2	eee3a83	link	true	/test check-dev2
idc-jenkins-ci-tidb/check_dev	eee3a83	link	true	/test check-dev

Full PR test history. Your PR dashboard.

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[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (release-7.5@1c99441). Click here to learn what that means.
The diff coverage is 100.0000%.

Additional details and impacted files

@@               Coverage Diff                @@
##             release-7.5     #51707   +/-   ##
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  Coverage               ?   71.9997%           
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  Files                  ?       1407           
  Lines                  ?     408968           
  Branches               ?          0           
================================================
  Hits                   ?     294456           
  Misses                 ?      94696           
  Partials               ?      19816           

Flag	Coverage Δ
unit	71.9997% <100.0000%> (?)

Flags with carried forward coverage won't be shown. Click here to find out more.

Components	Coverage Δ
dumpling	53.9913% <0.0000%> (?)
parser	∅ <0.0000%> (?)
br	53.3553% <0.0000%> (?)