Re-allow duplicate aliases within aliased JOINs.

Although the SQL spec forbids duplicate table aliases, historically
we've allowed queries like
    SELECT ... FROM tab1 x CROSS JOIN (tab2 x CROSS JOIN tab3 y) z
on the grounds that the aliased join (z) hides the aliases within it,
therefore there is no conflict between the two RTEs named "x".  The
LATERAL patch broke this, on the misguided basis that "x" could be
ambiguous if tab3 were a LATERAL subquery.  To avoid breaking existing
queries, it's better to allow this situation and complain only if
tab3 actually does contain an ambiguous reference.  We need only remove
the check that was throwing an error, because the column lookup code
is already prepared to handle ambiguous references.  Per bug #8444.

src/backend/parser/parse_clause.c

@@ -721,14 +721,15 @@ transformFromClauseItem(ParseState *pstate, Node *n,
 		 * we always push them into the namespace, but mark them as not
 		 * lateral_ok if the jointype is wrong.
 		 *
+		 * Notice that we don't require the merged namespace list to be
+		 * conflict-free.  See the comments for scanNameSpaceForRefname().
+		 *
 		 * NB: this coding relies on the fact that list_concat is not
 		 * destructive to its second argument.
 		 */
 		lateral_ok = (j->jointype == JOIN_INNER || j->jointype == JOIN_LEFT);
 		setNamespaceLateralState(l_namespace, true, lateral_ok);
 
-		checkNameSpaceConflicts(pstate, pstate->p_namespace, l_namespace);
-
 		sv_namespace_length = list_length(pstate->p_namespace);
 		pstate->p_namespace = list_concat(pstate->p_namespace, l_namespace);