Arms permitted when matching on uninhabited types

There seem to be two issues here:
```rust
pub enum Void {}

pub fn foo(x: Void) {
  match x {
    _ => {} // This arm shouldn't be permitted.
  };
  let _ = (); // This should be warned as unreachable, but isn't.
}
```
On the other hand, the following code *does* warn:
```rust
match () {
	() => {} // okay
	_ => {} // unreachable pattern
}
```