[cxx-interop] C++ reference types are not `AnyObject`s
C++ foreign reference types have custom reference counting mechanisms, so they cannot conform to `AnyObject`.
Currently Swift's type system treats C++ FRTs as `AnyObject`s on non-Darwin platforms, which is incorrect. This change makes sure the behavior is consistent with Darwin platform, i.e. a cast of C++ FRT to `AnyObject` is rejected by the typechecker.
rdar://136664617
(cherry picked from commit 82c076c9d35fc70be8c0be404d44db432897161f)
