Push attidentity and attgenerated handling into BuildDescForRelation()
Previously, this was handled by the callers separately, but it can be
trivially moved into BuildDescForRelation() so that it is handled in a
central place.
Reviewed-by: Alvaro Herrera <alvherre@alvh.no-ip.org>
Discussion: https://www.postgresql.org/message-id/flat/
52a125e4-ff9a-95f5-9f61-
b87cf447e4da@eisentraut.org