go/types: allows duplicate switch cases

go/types accepts the following program:

``` go
package a
func f() {
    switch 0 {
        case 0:
        case 0:
    }
}
```

While both gc and gccgo reject it with `duplicate case 0 in switch`.
Compilers should agree on validity of this program.

on commit 997b3545fd86c3a2d8e5fe6366174d7786e71278
