Fix(redshift): don't transform multi-arg DISTINCT clause

[georgesittas]

Redshift's aggregation function syntax generally expects [ DISTINCT | ALL ] expression, but LISTAGG is an exception: [DISTINCT] aggregate_expression [, 'delimiter' ].

I didn't address this as a special case, but rather made it so that Redshift doesn't transpile multi-arg DISTINCT expressions at all. The reason is that I played around and realized that Redshift probably doesn't support tuples ("records"?) in that context, so transpiling a query with a multi-arg DISTINCT expression to Redshift would yield invalid SQL anyway:

-- ERROR: could not identify an equality operator for type record
WITH tbl AS (
  SELECT 1 AS id, 'eggy' AS name UNION ALL SELECT NULL AS id, 'jake' AS name
)
SELECT COUNT(
  DISTINCT CASE WHEN id IS NULL THEN NULL WHEN name IS NULL THEN NULL ELSE (id, name) END
) AS cnt
FROM tbl

References:

• https://docs.aws.amazon.com/redshift/latest/dg/c_Aggregate_Functions.html
• https://docs.aws.amazon.com/redshift/latest/dg/r_LISTAGG.html