Overloads with single string literal type gives assignability errors

Sample code:

``` ts
interface Base {
    foo(p1: string, p2: string): void;
}

interface T extends Base {
    foo(p1: "p1" | "p2", p2: string): void; // Pass
}

interface T2 extends Base {
    foo(p1: "p3", p2: string): void; // Error: Specialized overload signature is not assignable to any non-specialized signature
}
```

Maybe related: #6272, however this seems stricter instead of looser
