When the planner creates a subquery as part of grouping planning, fix
the target list of the outer query to correctly refer to the outputs of
the subquery. Previously, the parse tree's target list was sometimes left
unchanged, so that it still contained Vars referring to the original
relations, which have been pushed down to the subquery. After adding
the subquery, the outer query only contains a single RTE for the subquery.

We usually got away with a bogus target list in the outer query, because
we had already decided how to construct the plan, and usually didn't look
at the original parse tree anymore. In some cases, though, when the caller
asked for it by passing 'use_root=true', we were already doing the right
thing, but otherwise we didn't bother. There's little downside to fixing
the target list, though, so let's do it unconditionally.

Fixes https://github.com/greenplum-db/gpdb/issues/6754Reviewed-by: NMelanie Plageman <mplageman@pivotal.io>