Tweak GROUP BY so that it will still accept result-column names, but only

after trying to resolve the item as an input-column name.  This allows us
to be compliant with the SQL92 spec for queries that fall within the spec,
while still accepting the same out-of-spec queries as 6.5 did.  You'll only
lose if there is an output column name that is the same as an input
column name, but doesn't refer to the same value.  7.0 will interpret
such a GROUP BY spec differently than 6.5 did.  No way around that, because
6.5 was clearly not spec compliant.

src/test/regress/expected/numerology.out

@@ -96,14 +96,18 @@ SELECT f1 AS two, max(f3) AS max_float, min(f3) as min_float
    2 |                    0 | -1.2345678901234e+200
 (2 rows)
 
--- Postgres used to accept this, but it is clearly against SQL92 to
--- interpret GROUP BY arguments as result column names; they should
--- be source column names *only*.  An error is expected.
+-- GROUP BY a result column name is not legal per SQL92, but we accept it
+-- anyway (if the name is not the name of any column exposed by FROM).
 SELECT f1 AS two, max(f3) AS max_float, min(f3) AS min_float
   FROM TEMP_GROUP
   GROUP BY two
   ORDER BY two, max_float, min_float;
-ERROR:  Attribute 'two' not found
+ two |      max_float       |       min_float       
+-----+----------------------+-----------------------
+   1 | 1.2345678901234e+200 |                     0
+   2 |                    0 | -1.2345678901234e+200
+(2 rows)
+
 SELECT f1 AS two, (max(f3) + 1) AS max_plus_1, (min(f3) - 1) AS min_minus_1
   FROM TEMP_GROUP
   GROUP BY f1

src/test/regress/sql/numerology.sql

@@ -70,9 +70,8 @@ SELECT f1 AS two, max(f3) AS max_float, min(f3) as min_float
   GROUP BY f1
   ORDER BY two, max_float, min_float;
 
--- Postgres used to accept this, but it is clearly against SQL92 to
--- interpret GROUP BY arguments as result column names; they should
--- be source column names *only*.  An error is expected.
+-- GROUP BY a result column name is not legal per SQL92, but we accept it
+-- anyway (if the name is not the name of any column exposed by FROM).
 SELECT f1 AS two, max(f3) AS max_float, min(f3) AS min_float
   FROM TEMP_GROUP
   GROUP BY two