pg_trigger.tgparentid

As mentioned in https://postgr.es/m/20191231194759.GA24692@alvherre.pgsql
I propose to add a new column to pg_trigger, which allows us to remove a
pg_depend scan when cloning triggers when adding/attaching partitions.
(It's not that I think the scan is a performance problem, but rather
than notionally we try not to depend on pg_depend contents for this kind
of semantic derivation.)

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Álvaro Herrera                            39°49'30"S 73°17'W