ddl: check foreign key constraint when drop,modify,change column

[crazycs520]

What problem does this PR solve?

• Check foreign key constraint when drop,modify,change column.

parser PR: pingcap/parser#675

Otherwise, select information_schema.KEY_COLUMN_USAGE will panic.

create table t1 (a int,b int);
create table t2 (c int,d int,foreign key (d) references t1 (b));
alter table t2 drop column d;
select count(*) from information_schema.KEY_COLUMN_USAGE;

What is changed and how it works?

Check List

Tests

• Unit test

Code changes

• Has exported function/method change

Side effects

Related changes

• Need to cherry-pick to the release branch

Release note

• Fix issue of select KEY_COLUMN_USAGE table panic cause by the foreign key.

[codecov]

Codecov Report

Merging #14043 into master will not change coverage.
The diff coverage is n/a.

@@             Coverage Diff             @@
##             master     #14043   +/-   ##
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  Coverage   80.2778%   80.2778%           
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  Files           483        483           
  Lines        121919     121919           
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  Hits          97874      97874           
  Misses        16287      16287           
  Partials       7758       7758

[AilinKid]

Rest LGTM

[reafans]

How about this， fristly，drop the constraint Foreign key in t.Meta().ForeignKeys，and then execut "alter table t4 drop column d"，it should be no err.

[crazycs520]

@reafans, Already add test for the case you said.

[reafans]

/run-all-tests

[reafans]

LGTM

[AilinKid]

LGTM

[zimulala]

LGTM

[sre-bot]

/run-all-tests

[sre-bot]

cherry pick to release-2.1 failed

[sre-bot]

cherry pick to release-3.0 failed