ddl add index on generated column with case-when expr will hang forever without cancel

[AilinKid]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

create table (
....
a generated always as (case when ....)
）
create index on a

2. What did you expect to see? (Required)

hang forever
the log is like below:
ddl add index error: UnkownType: ast.whenClause

3. What did you see instead (Required)

Even if we don't support the index column with case-when, it can also be cancelled by the DDL limit counts.

4. What is your TiDB version? (Required)

4.0.1 and previous version