cmd/yacc: fix debug print of token name

The array skips the first TOKSTART entries.

Fixes #4410.

R=golang-dev, ken2, ken
CC=golang-dev
https://golang.org/cl/6999054

src/cmd/yacc/units.y

@@ -11,6 +11,11 @@
 
 %{
 
+// This tag will end up in the generated y.go, so that forgetting
+// 'make clean' does not fail the next build.
+
+// +build ignore
+
 // units.y
 // example of a Go yacc program
 // usage is

src/cmd/yacc/yacc.go

@@ -3228,9 +3228,10 @@ type $$Lexer interface {
 const $$Flag = -1000
 
 func $$Tokname(c int) string {
-	if c > 0 && c <= len($$Toknames) {
-		if $$Toknames[c-1] != "" {
-			return $$Toknames[c-1]
+	// 4 is TOKSTART above
+	if c >= 4 && c-4 < len($$Toknames) {
+		if $$Toknames[c-4] != "" {
+			return $$Toknames[c-4]
 		}
 	}
 	return __yyfmt__.Sprintf("tok-%v", c)