Reorder joins before finalizing subqueries

[max-hoffman]

We cannot set field indexes for subquery expressions until the parent scope has been finalized.

The query that surfaced this bug was a subquery expression with a dependency on a join child, whose order is rearranged after we finalize subquery. Ex: the query below swaps the schema from under the subquery expression; y and is_one should match, but we reorder the join for a lookup, changing the scope schema from [xy,uv] -> [uv,xy]. The subquery expression now selects the wrong column for comparison:

> select y, (select 1 where y = 1) is_one
  from xy join uv on x = v
  order by y;
+---+--------+
| y | is_one |
+---+--------+
| 0 | 1      |
| 0 | 1      |
| 1 | NULL   |
| 1 | NULL   |
+---+--------+

We select the correct field now when the subquery is finalized after the parent scope is fixed:

> select y, (select 1 where y = 1) is_one
  from xy join uv on x = v
  order by y;
+---+--------+
| y | is_one |
+---+--------+
| 0 | NULL   |
| 0 | NULL   |
| 1 | 1      |
| 1 | 1      |
+---+--------+

[zachmu]

LGTM