Bug: 6g accepts fallthrough in type switch.

R=r
CC=go-dev
http://go/go-review/1016006

test/bugs/bug213.go

+// errchk $G $D/$F.go
+
+// Copyright 2009 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
+func main() {
+	var v interface{} = 0;
+	switch x := v.(type) {
+	case int:
+		fallthrough;		// ERROR "fallthrough"
+	default:
+		panic("fell through");
+	}
+}

test/golden.out

@@ -204,3 +204,6 @@ panic PC=xxx
 
 =========== bugs/bug212.go
 BUG: errchk: command succeeded unexpectedly
+
+=========== bugs/bug213.go
+BUG: errchk: command succeeded unexpectedly