pgsql: Build column mapping for grouping sets in all required cases.

Build column mapping for grouping sets in all required cases.

The previous coding frequently failed to fail because for one it's
unusual to have rollup clauses with one column, and for another
sometimes the wrong mapping didn't cause obvious problems.

Author: Jeevan Chalke
Reviewed-By: Andrew Gierth
Discussion: CAM2+6=W=9=hQOipH0HAPbkun3Z3TFWij_EiHue0_6UX=oR=1kw(at)mail(dot)gmail(dot)com
Backpatch: 9.5, where grouping sets were introduced

Branch
------
REL9_5_STABLE

Details
-------
http://git.postgresql.org/pg/commitdiff/65b86c1767a7dac0cc79dcfba7ba4cbd326dc03f

Modified Files
--------------
src/backend/optimizer/plan/planner.c | 9 ++-------
src/test/regress/expected/groupingsets.out | 23 +++++++++++++++++++++++
src/test/regress/sql/groupingsets.sql | 4 ++++
3 files changed, 29 insertions(+), 7 deletions(-)