Enforce child constraints during COPY TO a partitioned table.

Enforce child constraints during COPY TO a partitioned table.

The previous coding inadvertently checked the constraints for the
partitioned table rather than the target partition, which could
lead to data in a partition that fails to satisfy some constraint
on that partition.  This problem seems to date back to when
table partitioning was introduced; prior to that, there was only
one target table for a COPY, so the problem didn't occur, and the
code just didn't get updated.

Etsuro Fujita, reviewed by Amit Langote and Ashutosh Bapat

Discussion: https://postgr.es/message-id/5ABA4074.1090500%40lab.ntt.co.jp