IMPALA-1300: Allow subqueries in UNION operands

This enables the existing subquery rewrite rules to rewrite UNION
statements. UNION rewriting is easily done by simply calling the
rewriter for each operand in the UNION. At least one TPC-DS query
requires this functionality (IMPALA-1365).

The more difficult case of a UNION within a subquery is still not
supported.

Change-Id: I7f83eed0eb8ae81565e629f09f6918a4ba86ee13
Reviewed-on: http://gerrit.sjc.cloudera.com:8080/4859
Reviewed-by: Casey Ching <casey@cloudera.com>
Tested-by: jenkins