Support generic typedef

**TypeScript Version:**  2.7.0-dev.20171220

**Code**

```ts
/** @typedef {T | null | undefined} Maybe<T> */

/** @type Maybe<number> */
const x = 0;
```

**Expected behavior:**

No error.

**Actual behavior:**

```
a.js(1,15): error TS2304: Cannot find name 'T'.
a.js(3,11): error TS2315: Type 'Maybe' is not generic.
```