Allow an alias to be attached to a JOIN ... USING

This allows something like

    SELECT ... FROM t1 JOIN t2 USING (a, b, c) AS x

where x has the columns a, b, c and unlike a regular alias it does not
hide the range variables of the tables being joined t1 and t2.

Per SQL:2016 feature F404 "Range variable for common column names".

Reviewed-by: Vik Fearing <vik.fearing@2ndquadrant.com>
Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://www.postgresql.org/message-id/flat/454638cf-d563-ab76-a585-2564428062af@2ndquadrant.com

src/backend/parser/analyze.c

@@ -1744,6 +1744,7 @@ transformSetOperationStmt(ParseState *pstate, SelectStmt *stmt)
 										NIL,
 										NIL,
 										NULL,
+										NULL,
 										false);
 
 	sv_namespace = pstate->p_namespace;