[5.7] Support UNION aggregate queries

[staudenmeir]

Adds support for UNION aggregate queries:

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

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

This allows the pagination of UNION queries:

DB::table('posts')->union(DB::table('videos'))->orderBy('title')->paginate();

Simple queries like the first example work on SQL Server, pagination doesn't. SQL Server's OFFSET implementation makes this more compIex. I will leave that for another day.

Fixes #24941 and #2968.