DISTINCT aggregate crashes when ORDER BY repeats its argument

Hi,

This crashes the backend on master:

CREATE TABLE t (x text);
INSERT INTO t VALUES ('a'), ('a'), ('b');
SELECT array_agg(DISTINCT x ORDER BY x USING ~<~, x USING <)
FROM t;

I could repro it in all supported versions.

IIUC, the parser allows the same argument more than once in ORDER BY
with different sort operators. But we pick the Datum-sort path based
only on the number of inputs, so later sort keys get ignored.

For DISTINCT, we also mix up the number of inputs with the number of
DISTINCT keys..

I noticed this while looking at b715192e52e, which fixes the same
assumption in nodeSort.c. Attached is a patch for the aggregate case,
with tests for both paths. It uses Datum sorting only for one input
and one sort key, and allocates uniqslot for multiple DISTINCT keys.

Thoughts?

Regards,
Ayush