Swift: update comment

Author: redsun82

swift/extractor/visitors/VisitorBase.h

@@ -21,10 +21,10 @@ class VisitorBase {
 
 // we want to override the default swift visitor behaviour of chaining calls to immediate
 // superclasses by default and instead provide our own TBD default (using the exact type).
-// Moreover if the implementation class has translate##CLASS##KIND (that uses generated C++
-// classes), we want to use that. We detect that by comparing the member function pointer to the one
-// for a private undefined member function of the same name and signature in {Ast,Type}VisitorBase,
-// which will be shadowed (and thus different) in case the implementation class has it.
+// Moreover, if the implementation class has translate##CLASS##KIND (that uses generated C++
+// classes), we want to use that. We detect that by checking its return type. If it is different
+// from void (which is what is returned by a private unimplemented member function here) it means
+// we have implemented it in the visitor.
 #define DEFAULT(KIND, CLASS, PARENT)                                                              \
  public:                                                                                          \
   void visit##CLASS##KIND(swift::CLASS##KIND* e) {                                                \