A pass on #aggregateExpressionParserWith: should be done to match specs

[juliendelplanque]

See spec here: https://www.postgresql.org/docs/9.6/static/sql-expressions.html

[juliendelplanque]

Particularly, FILTER optional clauses are not implemented and aggregate_name ( [ expression [ , ... ] ] ) WITHIN GROUP ( order_by_clause ) [ FILTER ( WHERE filter_clause ) ] neither.