pg_dump failed to preserve the 'enabled' flag (which can be not only
disabled, but also REPLICA or ALWAYS) for partitions which had it
changed from their respective parents.  Attempt to handle that by
including a definition for such triggers in the dump, but replace the
standard CREATE TRIGGER line with an ALTER TRIGGER line.

Backpatch to 11, where these triggers can exist.  In branches 11 and 12,
pick up a few test lines from commit b9b408c4 to verify that
pg_upgrade is okay with these arrangements.
Co-authored-by: Justin Pryzby <pryzby@telsasoft.com>
Co-authored-by: Álvaro Herrera <alvherre@alvh.no-ip.org>
Discussion: https://postgr.es/m/20200930223450.GA14848@telsasoft.com