Fixed dmdfe generated class invariant calls. Also insert proper "this !is null" check before it. Fixed invariant* dstress regressions and generally seems like a good idea. Hope I didn't break anything.. minitests run fine!

#ifndef LDC_IR_IRDTYPE_H
#define LDC_IR_IRDTYPE_H

#include <set>

namespace llvm {
    class PATypeHolder;
}

struct IrDType
{
    IrDType();
    llvm::PATypeHolder* type;
};

#endif