Fix creation of duplicate foreign keys on partitions

When creating a foreign key in a partitioned table, if some partitions
already have equivalent constraints, we wastefully create duplicates of
the constraints instead of attaching to the existing ones.  That's
inconsistent with the de-duplication that is applied when a table is
attached as a partition.  To fix, reuse the FK-cloning code instead of
having a separate code path.

Backpatch to Postgres 11.  This is a subtle behavior change, but surely
a welcome one since there's no use in having duplicate foreign keys.

Discovered by Álvaro Herrera while thinking about a different problem
reported by Jesper Pedersen (bug #15587).

Author: Álvaro Herrera
Discussion: https://postgr.es/m/201901151935.zfadrzvyof4k@alvherre.pgsql

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