Add an assert to ArrayBuffer.getElementSlowPath.

The implementation is making a strong assumption about the array
element type (single-reference layout), but it is extremely
non-obvious that this helper is only called for those element types.

Swift SVN r30183

stdlib/public/core/ArrayBuffer.swift

@@ -372,6 +372,7 @@ extension _ArrayBuffer {
 
   @inline(never)
   func _getElementSlowPath(i: Int) -> AnyObject {
+    assert(_isClassOrObjCExistential(Element.self), "Only single reference elements can be indexed here.")
     let element: AnyObject
     if _isNative {
       // _isValidSubscript does no subscript checking for the slow path.