When an FK constraint is created, it needs the index on the referenced
table to exist and be valid.  When doing parallel pg_restore and the
referenced table was partitioned, this condition can sometimes not be
met, because pg_dump didn't emit sufficient object dependencies to
ensure so; this means that parallel pg_restore would fail in certain
conditions.  Fix by having pg_dump make the FK constraint object
dependent on the partition attachment objects for the constraint's
referenced index.

This has been broken since f56f8f8d, so backpatch to Postgres 12.

Discussion: https://postgr.es/m/20191005224333.GA9738@alvherre.pgsql