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
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REL_10_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/483882905a9a5dc72c9487ceee12320b9630ba2b

Modified Files
--------------
src/backend/optimizer/util/relnode.c | 4 +-
src/backend/parser/parse_relation.c | 103 +++++++++++++++++++++++------------
src/backend/parser/parse_target.c | 4 +-
src/backend/utils/adt/ruleutils.c | 4 +-
src/include/nodes/parsenodes.h | 5 ++
5 files changed, 79 insertions(+), 41 deletions(-)