reflect: make Value.Interface return immutable data

Fixes #3134.

R=golang-dev, r
CC=golang-dev
https://golang.org/cl/5713049
diff --git a/src/pkg/reflect/all_test.go b/src/pkg/reflect/all_test.go
index b21a99e..6bb0613 100644
--- a/src/pkg/reflect/all_test.go
+++ b/src/pkg/reflect/all_test.go
@@ -1743,3 +1743,15 @@
 		panic("zero Value")
 	}
 }
+
+func TestAlias(t *testing.T) {
+	x := string("hello")
+	v := ValueOf(&x).Elem()
+	oldvalue := v.Interface()
+	v.SetString("world")
+	newvalue := v.Interface()
+
+	if oldvalue != "hello" || newvalue != "world" {
+		t.Errorf("aliasing: old=%q new=%q, want hello, world", oldvalue, newvalue)
+	}
+}