Reset relhassubclass upon attaching table as a partition

Reset relhassubclass upon attaching table as a partition

We don't allow inheritance parents as partitions, and have checks to
prevent this; but if a table _was_ in the past an inheritance parents
and all their children are removed, the pg_class.relhassubclass flag
may remain set, which confuses the partition pruning code (most
obviously, it results in an assertion failure; in production builds it
may be worse.)

Fix by resetting relhassubclass on attach.

Backpatch to all supported versions.

Reported-by: Alexander Lakhin <[email protected]>
Reviewed-by: Tom Lane <[email protected]>
Discussion: https://postgr.es/m/18550-d5e047e9a897a889@postgresql.org