ddl: disallow dropping index on auto_increment column

[tangenta]

What problem does this PR solve?

drop table if exists t;
create table t (a int auto_increment, unique key (a));
alter table t drop index a;

TiDB:

Query OK, 0 rows affected
Time: 0.003s

Query OK, 0 rows affected
Time: 0.013s

Query OK, 0 rows affected
Time: 0.025s

MySQL 5.7:

Query OK, 0 rows affected
Time: 0.004s

Query OK, 0 rows affected
Time: 0.248s

(1075, 'Incorrect table definition; there can be only one auto column and it must be defined as a key')

What is changed and how it works?

When DropIndex(), check AutoIncrement flag in indexed columns.

Check List

Tests

• Unit test
• Integration test

Related changes

• Need to cherry-pick to the release branch

[codecov]

Codecov Report

Merging #11360 into master will decrease coverage by 0.2636%.
The diff coverage is 100%.

@@               Coverage Diff                @@
##             master     #11360        +/-   ##
================================================
- Coverage   81.5135%   81.2498%   -0.2637%     
================================================
  Files           423        423                
  Lines         91045      90170       -875     
================================================
- Hits          74214      73263       -951     
- Misses        11541      11597        +56     
- Partials       5290       5310        +20

[codecov]

Codecov Report

Merging #11360 into master will decrease coverage by 0.0991%.
The diff coverage is 100%.

@@               Coverage Diff                @@
##             master     #11360        +/-   ##
================================================
- Coverage   81.3368%   81.2376%   -0.0992%     
================================================
  Files           423        423                
  Lines         90392      90170       -222     
================================================
- Hits          73522      73252       -270     
- Misses        11577      11605        +28     
- Partials       5293       5313        +20

[zimulala]

LGTM

[winkyao]

LGTM

[ngaut]

LGTM

[ngaut]

/run-all-tests

[sre-bot]

cherry pick to release-3.0 failed