Issue Description:

The following code should print true, but it prints false instead.

func test() {
  let tooBigForInlineBuffer = (1, 2, 3, 4)
  var a: Any = tooBigForInlineBuffer
  withUnsafeMutablePointer(to: &a) {
    let p = UnsafeMutableRawPointer($0).assumingMemoryBound(to: AnyObject.self)
    print(isKnownUniquelyReferenced(&p.pointee))
  }
}
test()

Really there should be a first class way to detect uniqueness of existential boxes and another to access the lvalue stored in an existential without knowing its memory layout, but short of that, this technique ought to work.

Assigning to Standard Library for lack of a runtime component.