Fix handling of changed-Param signaling for CteScan plan nodes. We were using

the "cteParam" as a proxy for the possibility that the underlying CTE plan
depends on outer-level variables or Params, but that doesn't work very well
because it sometimes causes calling subqueries to be treated as SubPlans when
they could be InitPlans.  This is inefficient and also causes the outright
failure exhibited in bug #4902.  Instead, leave the cteParam out of it and
copy the underlying CTE plan's extParams directly.  Per bug #4902 from
Marko Tiikkaja.

src/test/regress/sql/with.sql

@@ -469,3 +469,19 @@ WITH RECURSIVE foo(i) AS
    UNION ALL
    SELECT (i+1)::numeric(10,0) FROM foo WHERE i < 10)
 SELECT * FROM foo;
+
+--
+-- test for bug #4902
+--
+with cte(foo) as ( values(42) ) values((select foo from cte));
+with cte(foo) as ( select 42 ) select * from ((select foo from cte)) q;
+
+-- test CTE referencing an outer-level variable (to see that changed-parameter
+-- signaling still works properly after fixing this bug)
+select ( with cte(foo) as ( values(f1) )
+         select (select foo from cte) )
+from int4_tbl;
+
+select ( with cte(foo) as ( values(f1) )
+          values((select foo from cte)) )
+from int4_tbl;