Class expressions should support in and out variance modifiers

[DanielRosenwasser]

Likely an oversight but

let Anon = class <out T> {
    foo(): InstanceType<(typeof Anon<T>)> {
        return this;
    }
}

let OuterC = class C<out T> {
    foo(): C<T> {
        return this;
    }
}

Results in the following message:

error! 'out' modifier can only appear on a type parameter of a class, interface or type alias

[ahejlsberg]

Yup, that's an oversight.

[ahejlsberg]

The repro actually reveals another problem: Validation of variance annotations needs to be deferred as it may otherwise cause circularity errors. Indeed, the Anon example errors if validation isn't deferred. I'll put up a PR that fixes both of these issues.