Parameter help for type parameters in a type

**TypeScript Version:**  3.1.0-dev.20180825

**Code**

```ts
class C<T, U> {}

new C</*a*/>;

let c: C</*b*/>;
```

**Expected behavior:**

Get type parameter help at both `/*a*/` and `/*b*/`.

**Actual behavior:**

Only get it at `/*a*/`.

See also `genericParameterHelpTypeReferences.ts`.