Modify the definition of window-function PARTITION BY and ORDER BY clauses

so that their elements are always taken as simple expressions over the
query's input columns.  It originally seemed like a good idea to make them
act exactly like GROUP BY and ORDER BY, right down to the SQL92-era behavior
of accepting output column names or numbers.  However, that was not such a
great idea, for two reasons:

1. It permits circular references, as exhibited in bug #5018: the output
column could be the one containing the window function itself.  (We actually
had a regression test case illustrating this, but nobody thought twice about
how confusing that would be.)

2. It doesn't seem like a good idea for, eg, "lead(foo) OVER (ORDER BY foo)"
to potentially use two completely different meanings for "foo".

Accordingly, narrow down the behavior of window clauses to use only the
SQL99-compliant interpretation that the expressions are simple expressions.

src/test/regress/sql/window.sql

@@ -167,8 +167,8 @@ SELECT count(*) OVER (PARTITION BY four) FROM (SELECT * FROM tenk1 UNION ALL SEL
 -- ordering by a non-integer constant is allowed
 SELECT rank() OVER (ORDER BY length('abc'));
 
--- but this draws an error: "ORDER BY 1" means order by first SELECT column
-SELECT rank() OVER (ORDER BY 1);
+-- can't order by another window function
+SELECT rank() OVER (ORDER BY rank() OVER (ORDER BY random()));
 
 -- some other errors
 SELECT * FROM empsalary WHERE row_number() OVER (ORDER BY salary) < 10;