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
This fixes a crash at runtime when destroying a Swift array of values of a C++ foreign reference type.
Swift optimizes the amount of metadata emitted for `_ContiguousArrayStorage<Element>` by reusing `_ContiguousArrayStorage<AnyObject>` whenever possible (see `getContiguousArrayStorageType`). However, C++ foreign reference types are not `AnyObject`s, since they have custom retain/release operations.
This change disables the `_ContiguousArrayStorage` metadata optimization for C++ reference types, which makes sure that `swift_arrayDestroy` will call the correct release operation for elements of `[MyCxxRefType]`.
rdar://127154770
