bug: constants should not be redeclarable

R=rsc
DELTA=9  (9 added, 0 deleted, 0 changed)
OCL=20187
CL=20187

test/bugs/bug126.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
+
+const none = 0  // same const identifier declared twice should not be accepted
+const none = 1  // ERROR "redeclared"

test/golden.out

@@ -173,6 +173,9 @@ BUG: errchk: command succeeded unexpectedly:  6g bugs/bug124.go
 =========== bugs/bug125.go
 BUG: errchk: command succeeded unexpectedly:  6g bugs/bug125.go
 
+=========== bugs/bug126.go
+BUG: errchk: command succeeded unexpectedly:  6g bugs/bug126.go
+
 =========== fixedbugs/bug016.go
 fixedbugs/bug016.go:7: overflow converting constant to uint