planner: update generated column inside a transaction should return error

[zz-jason]

What problem does this PR solve?

drop table if exists t;
create table t(a bigint, b bigint as (a+1));
begin;
insert into t(a) values(1);
update t set b=6 where b=7; -- got panic here
commit;

RCA:
Inside a transaction, DataSource is converted to LogicalUnionScan, we should also consider this case when we extract table alias name in update statement for virtual generated columns.

What is changed and how it works?

also check LogicalUnionScan, if it is, use its child to extract table alias name.

Check List

Tests

• Unit test

Code changes

• Has exported function/method change

Related changes

• Need to cherry-pick to the release branch
• Need to be included in the release note

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This change is 

[zz-jason]

/run-all-tests

[eurekaka]

LGTM

[alivxxx]

LGTM