pgsql: Enable BEFORE row-level triggers for partitioned tables

Enable BEFORE row-level triggers for partitioned tables

... with the limitation that the tuple must remain in the same
partition.

Reviewed-by: Ashutosh Bapat
Discussion: https://postgr.es/m/20200227165158.GA2071@alvherre.pgsql

Branch
------
master

Details
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https://git.postgresql.org/pg/commitdiff/487e9861d0cf83e9100ad0d0369147db3ef4ea73

Modified Files
--------------
doc/src/sgml/ref/create_trigger.sgml   |  2 +-
doc/src/sgml/trigger.sgml              |  3 +-
src/backend/commands/tablecmds.c       |  3 +-
src/backend/commands/trigger.c         | 40 +++++++++++------
src/backend/partitioning/partdesc.c    |  2 +-
src/include/utils/reltrigger.h         |  1 +
src/test/regress/expected/triggers.out | 78 ++++++++++++++++++++++++++++++++--
src/test/regress/sql/triggers.sql      | 55 +++++++++++++++++++++++-
8 files changed, 161 insertions(+), 23 deletions(-)