Fix ALTER TABLE .. ADD COLUMN with complex inheritance trees

Fix ALTER TABLE .. ADD COLUMN with complex inheritance trees

This command, when used to add a column on a parent table with a complex
inheritance tree, tried to update multiple times the same tuple in
pg_attribute for a child table when incrementing attinhcount, causing
failures with "tuple already updated by self" because of a missing
CommandCounterIncrement() between two updates.

This exists for a rather long time, so backpatch all the way down.

Reported-by: Alexander Lakhin
Author: Tender Wang
Reviewed-by: Richard Guo
Discussion: https://postgr.es/m/18297-b04cd83a55b51e35@postgresql.org
Backpatch-through: 12