[libc++][NFC] Fix experimental/type_traits namespace

[EricWF]

Does this header actually implement any of the V2 specification?

[GBuella]

In D72990#1828327, @EricWF wrote:

Does this header actually implement any of the V2 specification?

is_detected nonesuch and friends are all in V2, aren't they?

libcxx/include/experimental/type_traits

 /**
     experimental/type_traits synopsis
 
 // C++1y
 #include <type_traits>
 
 namespace std {
 namespace experimental {
-inline namespace fundamentals_v1 {
+inline namespace fundamentals_v2 {
 
   // 3.3.2, Other type transformations
   template <class> class invocation_type; // not defined
   template <class F, class... ArgTypes> class invocation_type<F(ArgTypes...)>;
   template <class> class raw_invocation_type; // not defined
   template <class F, class... ArgTypes> class raw_invocation_type<F(ArgTypes...)>;
 
   template <class T>
template <class Expected, template<class...> class Op, class... Args>
     constexpr bool is_detected_exact_v
       = is_detected_exact<Expected, Op, Args...>::value;
   template <class To, template<class...> class Op, class... Args>
      using is_detected_convertible = is_convertible<detected_t<Op, Args...>, To>;
   template <class To, template<class...> class Op, class... Args>
      constexpr bool is_detected_convertible_v
        = is_detected_convertible<To, Op, Args...>::value;
 
-} // namespace fundamentals_v1
+} // namespace fundamentals_v2
 } // namespace experimental
 } // namespace std
 
  */
 
 #include <experimental/__config>
 
 #if _LIBCPP_STD_VER > 11
 
 #include <initializer_list>
 #include <type_traits>
 
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #pragma GCC system_header
 #endif
 
-_LIBCPP_BEGIN_NAMESPACE_LFTS
+_LIBCPP_BEGIN_NAMESPACE_LFTS_V2
 
 // 3.3.2, Other type transformations
 /*
 template <class>
 class _LIBCPP_TEMPLATE_VIS raw_invocation_type;
 
 template <class _Fn, class ..._Args>
 class _LIBCPP_TEMPLATE_VIS raw_invocation_type<_Fn(_Args...)>;
template <class Expected, template<class...> class _Op, class... _Args>
   _LIBCPP_CONSTEXPR bool is_detected_exact_v = is_detected_exact<Expected, _Op, _Args...>::value;
 
 template <class To, template<class...> class _Op, class... _Args>
   using is_detected_convertible = is_convertible<detected_t<_Op, _Args...>, To>;
 template <class To, template<class...> class _Op, class... _Args>
   _LIBCPP_CONSTEXPR bool is_detected_convertible_v = is_detected_convertible<To, _Op, _Args...>::value;
 
 
-_LIBCPP_END_NAMESPACE_LFTS
+_LIBCPP_END_NAMESPACE_LFTS_V2
 
 #endif /* _LIBCPP_STD_VER > 11 */
 
 #endif /* _LIBCPP_EXPERIMENTAL_TYPE_TRAITS */