Refactor ATExecAddColumn() to use BuildDescForRelation()

Refactor ATExecAddColumn() to use BuildDescForRelation()

BuildDescForRelation() has all the knowledge for converting a
ColumnDef into pg_attribute/tuple descriptor.  ATExecAddColumn() can
make use of that, instead of duplicating all that logic.  We just pass
a one-element list of ColumnDef and we get back exactly the data
structure we need.  Note that we don't even need to touch
BuildDescForRelation() to make this work.

Reviewed-by: Alvaro Herrera <alvherre@alvh.no-ip.org>
Discussion: https://www.postgresql.org/message-id/flat/52a125e4-ff9a-95f5-9f61-b87cf447e4da@eisentraut.org