Any number can be assigned to enum member type

**Search Terms:**
enum assign number

**Code**

```ts
enum X {
  A, B
}

const x: X.A = 5;

const y: X.A = X.B;
```

**Expected behavior:**
2 errors: when assigning number literal and wrong enum member.

**Actual behavior:**
Typescript allows assigning any number to specific member type.

**Playground Link:**
http://bit.ly/2CZsUrK
