r? @nikomatsakis

(Related issue about `debug_tuple` at rust-lang/rfcs#1448.)

```rust

fn main() {
    let _x = ();
}
```

```diff
--- empty_tuple-old.mir	2016-01-06 16:04:24.206409186 -0600
+++ empty_tuple-new.mir	2016-01-06 14:26:17.324888585 -0600
@@ -1,13 +1,13 @@
 fn() -> () {
     let var0: (); // _x
     let mut tmp0: ();

     bb0: {
-        var0 = ;
+        var0 = ();
         Some(goto -> bb1);
     }

     bb1: {
         Some(return);
     }
 }
```