pgsql: Prevent setting NO INHERIT on partitioned NOT NULL constraints

Prevent setting NO INHERIT on partitioned NOT NULL constraints

The documentation states that NOT NULL constraints on partitioned tables
are always inherited by all partitions, and therefore cannot be declared
NO INHERIT. While a check already existed to reject creating such
constraints with NO INHERIT, previously the same check was missing for
ALTER TABLE ... ALTER CONSTRAINT ... NO INHERIT.

This commit adds the missing check so that attempting to set NO INHERIT
on a partitioned NOT NULL constraint now fails.

Backpatch to v18, where ALTER TABLE ... ALTER CONSTRAINT ... [NO] INHERIT
was added.

Author: Andreas Karlsson <andreas(at)proxel(dot)se>
Reviewed-by: Jim Jones <jim(dot)jones(at)uni-muenster(dot)de>
Reviewed-by: Fujii Masao <masao(dot)fujii(at)gmail(dot)com>
Discussion: https://postgr.es/m/ecc985ad-6ec1-4094-a315-317943ca5f3f@proxel.se
Backpatch-through: 18

Branch
------
REL_18_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/41247cdf695b99eb4b6359ef3d6bdcfbad321847

Modified Files
--------------
src/backend/commands/tablecmds.c | 6 ++++++
src/test/regress/expected/constraints.out | 4 ++++
src/test/regress/sql/constraints.sql | 3 +++
3 files changed, 13 insertions(+)