Fix crash-on-invalid when calling non-required init on a metatype

[slavapestov]

We were missing some checks. Reported by a user on Twitter.

[slavapestov]

@swift-ci Please smoke test

[slavapestov]

@swift-ci Please test source compatibility

[rudkx]

Do we have any tests that use 'super.init'?

From a brief glance it's not clear to me that the right thing will necessarily happen in that case.

[slavapestov]

Do you mean super.init from inside a static method? I’ll try it out. It should probably be banned

[slavapestov]

@rudkx This works right now:

public class C {
  init() {}
}

public class D : C {
  static func f() -> D {
    return super.init()
  }
}

It emits a statically-dispatched call to C.init, just like if you had written return C(). I can add a test case for this in a separate PR if you feel its necessary.

[rudkx]

I only brought it up because you deleted a line of code checking explicitly for SuperRefExpr and it wasn't clear to me that handling it correctly would just fall out from the remaining code. I'll leave it to your best judgement on whether we're already testing adequately for that.