Divided ir/irtypestruct.* in general aggregate and struct specific part.

This is just to improve clarity, as it was rather non-obvious
what of the code also applied to classes before.

IrTypeAggr::createInitializerConstant would currently belong in
IrTypeStruct, but this will be refactored anyway.

gen/tollvm.cpp

@@ -31,6 +31,7 @@
 #include "ir/irtype.h"
 #include "ir/irtypeclass.h"
 #include "ir/irtypefunction.h"
+#include "ir/irtypestruct.h"
 
 bool DtoIsPassedByRef(Type* type)
 {