pgsql: 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(at)alvh(dot)no-ip(dot)org>
Discussion: https://www.postgresql.org/message-id/flat/52a125e4-ff9a-95f5-9f61-b87cf447e4da(at)eisentraut(dot)org

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/a1604237a6ffee70b171bacd5f36b0e380afd33a

Modified Files
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src/backend/commands/tablecmds.c | 89 ++++++++++------------------------------
1 file changed, 22 insertions(+), 67 deletions(-)