cast overly permissive with extern(C++ ) classes; should cast through `void*`

[Timothee Cour]

should we force force casting through `void*` for extern(C++) classes ?
i.e. `cast(Derived) cast(void*) base_instance;`

currently, `cast(A) unrelad_cpp_instance` happily compiles but
shouldn't, it's very error prone even though the cast syntax suggests
type safety; especially a problem if we make a class become
extern(C++) in the future

[came up after discussing with jacob-carlborg]