pgsql: Provide the OR REPLACE option for CREATE TRIGGER.

Provide the OR REPLACE option for CREATE TRIGGER.

This is mostly straightforward.  However, we disallow replacing
constraint triggers or changing the is-constraint property; perhaps
that can be added later, but the complexity versus benefit tradeoff
doesn't look very good.

Also, no special thought is taken here for whether replacing an
existing trigger should result in changes to queued-but-not-fired
trigger actions.  We just document that if you're surprised by the
results, too bad, don't do that.  (Note that any such pending trigger
activity would have to be within the current session.)

Takamichi Osumi, reviewed at various times by Surafel Temesgen,
Peter Smith, and myself

Discussion: https://postgr.es/m/0DDF369B45A1B44B8A687ED43F06557C010BC362@G01JPEXMBYT03

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/92bf7e2d027466d750b4ac5b026f6f4ac29be881

Modified Files
--------------
doc/src/sgml/ref/create_trigger.sgml   |  76 +++++++++-----
src/backend/catalog/index.c            |   7 +-
src/backend/commands/tablecmds.c       |  28 +++---
src/backend/commands/trigger.c         | 174 +++++++++++++++++++++++----------
src/backend/nodes/copyfuncs.c          |   3 +-
src/backend/nodes/equalfuncs.c         |   3 +-
src/backend/parser/gram.y              |  52 +++++-----
src/include/nodes/parsenodes.h         |   3 +-
src/test/regress/expected/triggers.out |  89 +++++++++++++++++
src/test/regress/sql/triggers.sql      |  87 +++++++++++++++++
10 files changed, 408 insertions(+), 114 deletions(-)