Fix LATERAL references to target table of UPDATE/DELETE.

I failed to think much about UPDATE/DELETE when implementing LATERAL :-(.
The implemented behavior ended up being that subqueries in the FROM or
USING clause (respectively) could access the update/delete target table as
though it were a lateral reference; which seems fine if they said LATERAL,
but certainly ought to draw an error if they didn't.  Fix it so you get a
suitable error when you omit LATERAL.  Per report from Emre Hasegeli.

src/backend/parser/parse_clause.c

@@ -204,6 +204,10 @@ setTargetTable(ParseState *pstate, RangeVar *relation,
 
 	/*
 	 * If UPDATE/DELETE, add table to joinlist and namespace.
+	 *
+	 * Note: some callers know that they can find the new ParseNamespaceItem
+	 * at the end of the pstate->p_namespace list.  This is a bit ugly but not
+	 * worth complicating this function's signature for.
 	 */
 	if (alsoSource)
 		addRTEtoQuery(pstate, rte, true, true, true);