Issue Description:

Objective-C has some classes that don't support the ObjC weak-reference machinery, and therefore Clang warns about making a __weak reference to such a class. Swift's weak and unowned references depend on that same machinery for ObjC classes, so we should have a similar diagnostic; I think ours should just be a hard error, though.

Note that attributes like IBOutlet imply making a weak reference, and we should make sure the diagnostic catches them as well. Or maybe we should just make the outlet implicitly unowned(unsafe)?

The code from the Clang compiler that performs this check is as follows; this bug is mostly a matter of doing a similar check when checking a weak/unowned reference in Swift.

  if (lifetime == Qualifiers::OCL_Weak) {
    if (const ObjCObjectPointerType *ObjT =
          type->getAs<ObjCObjectPointerType>()) {
      if (ObjCInterfaceDecl *Class = ObjT->getInterfaceDecl()) {
        if (Class->isArcWeakrefUnavailable()) {
          S.Diag(AttrLoc, diag::err_arc_unsupported_weak_class);
          S.Diag(ObjT->getInterfaceDecl()->getLocation(),
                 diag::note_class_declared);
        }
      }
    }
  }