pgsql: Fix plan created for inherited UPDATE/DELETE with all tables exc

Fix plan created for inherited UPDATE/DELETE with all tables excluded.

In the case where inheritance_planner() finds that every table has
been excluded by constraints, it thought it could get away with
making a plan consisting of just a dummy Result node. While certainly
there's no updating or deleting to be done, this had two user-visible
problems: the plan did not report the correct set of output columns
when a RETURNING clause was present, and if there were any
statement-level triggers that should be fired, it didn't fire them.

Hence, rather than only generating the dummy Result, we need to
stick a valid ModifyTable node on top, which requires a tad more
effort here.

It's been broken this way for as long as inheritance_planner() has
known about deleting excluded subplans at all (cf commit 635d42e9c),
so back-patch to all supported branches.

Amit Langote and Tom Lane, per a report from Petr Fedorov.

Discussion: https://postgr.es/m/5da6f0f0-1364-1876-6978-907678f89a3e@phystech.edu

Branch
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REL9_6_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/714344ae68338add0ccd82f3b3564344ec0e8a58

Modified Files
--------------
src/backend/optimizer/plan/planner.c | 66 +++++++++++++++++++++++-----------
src/test/regress/expected/inherit.out | 41 +++++++++++++++++++++
src/test/regress/expected/triggers.out | 34 ++++++++++++++++++
src/test/regress/sql/inherit.sql | 15 ++++++++
src/test/regress/sql/triggers.sql | 27 ++++++++++++++
5 files changed, 162 insertions(+), 21 deletions(-)