gc: improve error message for composite literals with unexpected newlines

R=golang-dev, r, rsc
CC=golang-dev
https://golang.org/cl/5857045

src/cmd/gc/go.errors

@@ -65,6 +65,9 @@ static struct {
 	% loadsys package imports LVAR LNAME '=' LNAME '{' LNAME ';'
 	"need trailing comma before newline in composite literal",
 	
+	% loadsys package imports LVAR LNAME '=' comptype '{' LNAME ';'
+	"need trailing comma before newline in composite literal",
+	
 	% loadsys package imports LFUNC LNAME '(' ')' '{' LFUNC LNAME
 	"nested func not allowed",
 

src/cmd/gc/yerr.h

@@ -65,6 +65,9 @@ static struct {
 	425, ';',
 	"need trailing comma before newline in composite literal",
 	
+	435, ';',
+	"need trailing comma before newline in composite literal",
+	
 	112, LNAME,
 	"nested func not allowed",
 

test/syntax/composite.go

+// errorcheck
+
+// Copyright 2012 The Go Authors.  All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package main
+
+var a = []int{
+	3 // ERROR "need trailing comma before newline in composite literal"
+}