Allow extends type to be supertype of constraint types when narrowing conditional return type

[gabritto]

Before, we would not attempt to do return type narrowing for the following conditional type:

function voidRet<T extends { a: string } | undefined>(x: T): T extends {} ? void : T extends undefined ? number : never {
    if (x) {
        return;
    }
    return 1;
}

The reason was that the extends type {} was not identical to any element of the parameter union constraint type { a: string } | undefined.
This PR lifts the restriction, only requiring that a conditional's extends type be a supertype of an element of the parameter union constraint type. So now TS will attempt return type narrowing for the above example.

Note that just like the user is responsible for making sure the union components of the parameter type are disjoint types, the extends types in the conditional type also should be disjoint, or bad things can happen (see second example in the new test).

[ehoogeveen-medweb]

Out of curiosity, does the following (object instead of {}) also work after this change?

function voidRet<T extends { a: string } | undefined>(x: T): T extends object ? void : T extends undefined ? number : never {
    if (x) {
        return;
    }
    return 1;
}