Add missing override keyword to virtual functions

Add missing override markers to the following functions of the C++
runtime:

  - TokensStartState::getStateType()
  - TagChunk::toString()
  - TextChunk::toString()

The missing markers made builds against the API with -Wsuggest-override
choke.

runtime/Cpp/runtime/src/atn/TokensStartState.h

@@ -14,7 +14,7 @@ namespace atn {
   class ANTLR4CPP_PUBLIC TokensStartState final : public DecisionState {
 
   public:
-    virtual size_t getStateType();
+    virtual size_t getStateType() override;
   };
 
 } // namespace atn

runtime/Cpp/runtime/src/tree/pattern/TagChunk.h

@@ -70,7 +70,7 @@ namespace pattern {
     /// are returned in the form {@code label:tag}, and unlabeled tags are
     /// returned as just the tag name.
     /// </summary>
-    virtual std::string toString();
+    virtual std::string toString() override;
 
   private:
     /// This is the backing field for <seealso cref="#getTag"/>.

runtime/Cpp/runtime/src/tree/pattern/TextChunk.h

@@ -43,7 +43,7 @@ namespace pattern {
     /// The implementation for <seealso cref="TextChunk"/> returns the result of
     /// <seealso cref="#getText()"/> in single quotes.
     /// </summary>
-    virtual std::string toString();
+    virtual std::string toString() override;
   };
 
 } // namespace pattern