Improve generated column names for cases involving sub-SELECTs.

We'll now use "exists" for EXISTS(SELECT ...), "array" for ARRAY(SELECT
...), or the sub-select's own result column name for a simple expression
sub-select.  Previously, you usually got "?column?" in such cases.

Marti Raudsepp, reviewed by Kyotaro Horiugchi

src/test/regress/expected/aggregates.out

@@ -300,9 +300,9 @@ LINE 4:                where sum(distinct a.four + b.four) = b.four)...
 select
   (select max((select i.unique2 from tenk1 i where i.unique1 = o.unique1)))
 from tenk1 o;
- ?column? 
-----------
-     9999
+ max  
+------
+ 9999
 (1 row)
 
 --

src/test/regress/expected/subselect.out

@@ -490,20 +490,20 @@ select view_a from view_a;
 (1 row)
 
 select (select view_a) from view_a;
- ?column? 
-----------
+ view_a 
+--------
  (42)
 (1 row)
 
 select (select (select view_a)) from view_a;
- ?column? 
-----------
+ view_a 
+--------
  (42)
 (1 row)
 
 select (select (a.*)::text) from view_a a;
- ?column? 
-----------
+  a   
+------
  (42)
 (1 row)
 

src/test/regress/expected/with.out

@@ -1065,7 +1065,7 @@ with cte(foo) as ( select 42 ) select * from ((select foo from cte)) q;
 select ( with cte(foo) as ( values(f1) )
          select (select foo from cte) )
 from int4_tbl;
-  ?column?   
+     foo     
 -------------
            0
       123456
@@ -1077,7 +1077,7 @@ from int4_tbl;
 select ( with cte(foo) as ( values(f1) )
           values((select foo from cte)) )
 from int4_tbl;
-  ?column?   
+   column1   
 -------------
            0
       123456