Multi-part `ALTER TABLE` statements can't reference columns added in the same statement.

This works in MySQL (and in the GMS in-memory DB):

```
create table t1(pk int primary key);
alter table t1 add column b int, add column c int as (b+1);
```

But trying it in Dolt results in an error:

```
column "b" could not be found in any table in scope
```

Most likely this happens because we attempt to resolve all parts of the statement before any of the new columns are added.