Fix propagation of persistence to sequences in ALTER TABLE / ADD COLUMN

Commit 344d62fb9a9 (2022) introduced unlogged sequences and made it so 
that identity/serial sequences automatically get the persistence level 
of their owning table.  But this works only for CREATE TABLE and not for 
ALTER TABLE / ADD COLUMN.  This patch fixes that.  (should be 
backpatched to 15, 16)