Issue Description:

I would expect the following function to return `5` if called with `Int.self`, and `nil` otherwise.

It doesn't compile, because the rhs of `as?` is evaluated more like a declaration than the expression it is.

func foo<T>(_ ty: T.Type) -> T? {
  let x : Any = 5
  let y = x as? ty // error: Use of undeclared type 'ty'
  return y
}

I suppose this is on the border of being a language improvement, rather than a bug, due to the duration this has existed and the differences between declaration type references and expression type references, but I still believe this is a bug.