[DF] Detection of types not instantiatable by the interpreter has broken edge cases

First reported on the forum here.

The problem is that in RInterface::DefineImpl we take the output of TypeID2TypeName as an indicator of whether a given type is known to the interpreter or not. In some cases, e.g. vector<UnknownType>, TClass::GetClass and therefore TypeID2TypeName return the name of the type even if it's not instantiatable by the interpreter.

DefineImpl should further check the type_info. Philippe suggests the following logic:

std::string CheckTypeID(const std::type_info &t) {

   int err = 0;

   char* demangled_name = TClassEdit::DemangleTypeIdName(t, err);

   if (err) return "Unknown typeid";

   ClassInfo_t *info = gInterpreter->ClassInfo_Factory(demangled_name);

   if  ( info && gInterpreter->ClassInfo_IsValid(info) )

      return gInterpreter->ClassInfo_FullName(info);

   else

      return "Unknown typeid";

}