Overloads with singleton string literal types have assignability issues

[DanielRosenwasser]

The following compiles:

function f(x: "foo"): number;
function f(x: string): number {
    return 0;
}

function g(x: "bar"): number;
function g(x: string): number {
    return 0;
}

var a = f;
var b = g;

a = b;
b = a;

This is because specialized overloads are ignored for assignment compatibility purposes.

Related to #6143.