Fix vararg ABI

Requires https://github.com/ldc-developers/ldc/pull/768 and addresses https://github.com/ldc-developers/ldc/issues/702 and https://github.com/ldc-developers/ldc/issues/73 (for Win64 only!).
Okay, I see the problem. We should probably fix this at some point by moving `__va_list` into `object` and properly initializing `Type::tvalist`, but that can wait for now.