When null_ordered_last is enabled (or an explicit NULLS FIRST/LAST is given),
buildOrderBy synthesizes a (col IS NULL) ordering column ahead of each column
with non-default NULLS ordering. For a plain SELECT DISTINCT, constructDistinct
rejected any ordering column not in the select list, so these synthesized
columns triggered a spurious

  for SELECT DISTINCT, ORDER BY expressions must appear in select list

error. The DISTINCT ON path (buildDistinctOn) already tolerates these columns
(case 3): a (col IS NULL) expression is functionally determined by col, so when
col is a grouping column the synthesized column can be treated as one too
without changing the result.

This extracts that check into a shared isSynthesizedNullsOrderingCol helper and
uses it from both constructDistinct and buildDistinctOn. It fixes user queries
such as

  SET null_ordered_last = on;
  SELECT DISTINCT a FROM t ORDER BY a;

as well as DROP USER and DROP ROLE, which internally run a
SELECT DISTINCT ... ORDER BY against system.privileges. Fixing constructDistinct
rather than suppressing the session setting for internal queries is the correct
layer, since the error reproduces with the plain user query above.

Fixes cockroachdb#168317
Epic: none

Release note (bug fix): Fixed a bug that caused SELECT DISTINCT ... ORDER BY
queries to fail with a "for SELECT DISTINCT, ORDER BY expressions must appear
in select list" error when the null_ordered_last session setting was enabled.
The same error caused DROP USER and DROP ROLE statements to fail, since they
run such a query internally.