code: public SupertypeQueryResults getSupertypeQueryResults(ClassDescriptor classDescriptor) {
          SupertypeQueryResults supertypeQueryResults = supertypeSetMap.get(classDescriptor);
          if (supertypeQueryResults == null) {
              supertypeQueryResults = computeSupertypes(classDescriptor);
              supertypeSetMap.put(classDescriptor, supertypeQueryResults);
          }
          return supertypeQueryResults;
      }
docs: Look up or compute the SupertypeQueryResults for class named by given
  ClassDescriptor.
  
  @param classDescriptor
  a ClassDescriptor
  @return SupertypeQueryResults for the class named by the ClassDescriptor