pgsql: Attach ON CONFLICT SET ... WHERE to the correct planstate.

Attach ON CONFLICT SET ... WHERE to the correct planstate.

The previous coding was a leftover from attempting to hang all the on
conflict logic onto modify table's child nodes. It appears to not have
actually caused problems except for explain.

Add test exercising the broken and some other code paths.

Author: Peter Geoghegan and Andres Freund

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/e4942f7a56efcfaabed5db7bde29ee21bef2f6e2

Modified Files
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src/backend/executor/nodeModifyTable.c        |    2 +-
src/test/regress/expected/insert_conflict.out |   16 ++++++++++++++++
src/test/regress/sql/insert_conflict.sql      |    2 ++
3 files changed, 19 insertions(+), 1 deletion(-)