pgsql: 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 <exclusion@gmail.com>
Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/18550-d5e047e9a897a889@postgresql.org

Branch
------
REL_16_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/084814d888b163b20ab26674108363e3ef179769

Modified Files
--------------
src/backend/catalog/heap.c                |  8 ++++++++
src/test/regress/expected/alter_table.out | 10 +++++++++-
src/test/regress/sql/alter_table.sql      |  7 +++++++
3 files changed, 24 insertions(+), 1 deletion(-)