[5.7] Fix UNION aggregate queries with columns

[staudenmeir]

UNION aggregate queries don't apply the selected columns to the base query:

DB::table('posts')->select('id')->union(
  DB::table('videos')->select('id')
)->count()

# expected
select count(*) as aggregate from (
  (select `id` from `posts`) union (select `id` from `videos`)
) as `temp_table`

# actual
select count(*) as aggregate from (
  (select * from `posts`) union (select `id` from `videos`)
          ^
) as `temp_table`

The selected columns are irrelevant for the result, but the UNION query doesn't work with different numbers of columns ("cardinality violation").

This also affects pagination queries.

Fixes #26444.