pgsql: Clean up handling of dropped columns in NAMEDTUPLESTORE RTEs.

Clean up handling of dropped columns in NAMEDTUPLESTORE RTEs.

The NAMEDTUPLESTORE patch piggybacked on the infrastructure for
TABLEFUNC/VALUES/CTE RTEs, none of which can ever have dropped columns,
so the possibility was ignored most places. Fix that, including adding a
specification to parsenodes.h about what it's supposed to look like.

In passing, clean up assorted comments that hadn't been maintained
properly by said patch.

Per bug #14799 from Philippe Beaudoin. Back-patch to v10.

Discussion: https://postgr.es/m/20170906120005.25630.84360@wrigleys.postgresql.org

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/8689e38263af7755b8100203e325a5953ed1e602

Modified Files
--------------
src/backend/optimizer/util/relnode.c | 4 +-
src/backend/parser/parse_relation.c | 98 ++++++++++++++++++++++++------------
src/backend/parser/parse_target.c | 4 +-
src/backend/utils/adt/ruleutils.c | 4 +-
src/include/nodes/parsenodes.h | 5 ++
5 files changed, 77 insertions(+), 38 deletions(-)