[Reg 2.078] casting from C++ class to Object always succeds

While working on #23262 I noticed that casting a C++ (or Objective-C) class instance to Object succeeds, but should result in null:
```D
extern(C++) class Cpp {}

void main()
{
    auto cpp = new Cpp;
    assert(cast(Object)cpp is null);
}
```
According to run.dlang.io this fails since dmd 2.078.
