Fix parser so that we don't modify the user-written ORDER BY list in order

Fix parser so that we don't modify the user-written ORDER BY list in order
to represent DISTINCT or DISTINCT ON.  This gets rid of a longstanding
annoyance that a view or rule using SELECT DISTINCT will be dumped out
with an overspecified ORDER BY list, and is one small step along the way
to decoupling DISTINCT and ORDER BY enough so that hash-based implementation
of DISTINCT will be possible.  In passing, improve transformDistinctClause
so that it doesn't reject duplicate DISTINCT ON items, as was reported by
Steve Midgley a couple weeks ago.