Completions in switch: don't include already-covered cases

**TypeScript Version:**  nightly (2.2.0-dev.20170126)

**Code**

```ts
/// <reference path='fourslash.ts'/>

////enum E { A, B }
////declare const e: E;
////switch (e) {
////    case E.A:
////        return 0;
////    case E./**/
////}

verify.completionsAt("", ["B"]);
```

Currently we get get both `A` and `B`.