update tuple.h

as discussed with @rvermeulen

Author: jeongsoolee09

cpp/common/test/includes/standard-library/tuple.h

@@ -1,12 +1,11 @@
 namespace std {
 template <class... Types> class tuple {};
 template <class... Types> std::tuple<Types...> make_tuple(Types &&...args);
-// TODO change this to example from cpp standard
 struct ignore_t {
   template <typename T>
   constexpr // required since C++14
       void
       operator=(T &&) const noexcept {}
 };
-inline constexpr std::ignore_t ignore; // 'const' only until C++17
+inline const std::ignore_t ignore; // 'const' only until C++17
 } // namespace std