Fixed a few warnings.

runtime/Cpp/runtime/src/atn/ArrayPredictionContext.cpp

@@ -78,7 +78,7 @@ bool ArrayPredictionContext::operator == (PredictionContext const& o) const {
     antlrcpp::Arrays::equals(parents, other->parents);
 }
 
-std::string ArrayPredictionContext::toString() {
+std::string ArrayPredictionContext::toString() const {
   if (isEmpty()) {
     return "[]";
   }

runtime/Cpp/runtime/src/atn/ArrayPredictionContext.h

@@ -62,7 +62,7 @@ namespace atn {
     virtual int getReturnState(size_t index) const override;
     bool operator == (const PredictionContext &o) const override;
 
-    virtual std::string toString();
+    virtual std::string toString() const override;
   private:
     std::vector<Ref<PredictionContext>> makeRef(const std::vector<std::weak_ptr<PredictionContext>> &input);
   };

runtime/Cpp/runtime/src/tree/ParseTree.h

@@ -67,6 +67,7 @@ namespace tree {
     /// 	based upon the parser.
     /// </summary>
     virtual std::string toStringTree(Parser *parser) = 0;
+    using Tree::toStringTree;
   };
 
 } // namespace tree

runtime/Cpp/runtime/src/tree/ParseTreeVisitor.h

@@ -45,13 +45,14 @@ namespace tree {
   /// operations with no return type. </param>
   template<typename T>
   class ANTLR4CPP_PUBLIC ParseTreeVisitor {
-
+  public:
+    virtual ~ParseTreeVisitor() {}
+    
     /// <summary>
     /// Visit a parse tree, and return a user-defined result of the operation.
     /// </summary>
     /// <param name="tree"> The <seealso cref="ParseTree"/> to visit. </param>
     /// <returns> The result of visiting the parse tree. </returns>
-  public:
     virtual T* visit(ParseTree *tree) = 0;
 
     /// <summary>