[demangling] make printGenericSignature virtual

(cherry picked from commit 6324f20)

Author: charles-zablit

include/swift/Demangling/Demangle.h

@@ -949,7 +949,7 @@ class NodePrinter {
 
   void printImplFunctionType(NodePointer fn, unsigned depth);
 
-  void printGenericSignature(NodePointer Node, unsigned depth);
+  virtual void printGenericSignature(NodePointer Node, unsigned depth);
 
   void printFunctionSigSpecializationParams(NodePointer Node, unsigned depth);