ICE with const as statement

```
const FOO: i32 = 42;
fn main() {
    FOO;
}
```

```
<anon>:3:5: 3:9 warning: path statement with no effect, #[warn(path_statements)] on by default
<anon>:3     FOO;
             ^~~~
<anon>:3:5: 3:8 error: internal compiler error: constant expression should not reach expr::trans_def
<anon>:3     FOO;
             ^~~
```

Note that this is fixed by changing to `let _ = FOO;`.
