libstdc++: Do not use __is_convertible unconditionally [PR113241]

The new __is_convertible built-in should only be used after checking
that it's supported.

libstdc++-v3/ChangeLog:

	PR libstdc++/113241
	* include/std/type_traits (is_convertible_v): Guard use of
	built-in with preprocessor check.

libstdc++-v3/include/std/type_traits

@@ -3477,8 +3477,13 @@ template <typename _Tp>
 #endif
 template <typename _Base, typename _Derived>
   inline constexpr bool is_base_of_v = __is_base_of(_Base, _Derived);
+#if _GLIBCXX_USE_BUILTIN_TRAIT(__is_convertible)
 template <typename _From, typename _To>
   inline constexpr bool is_convertible_v = __is_convertible(_From, _To);
+#else
+template <typename _From, typename _To>
+  inline constexpr bool is_convertible_v = is_convertible<_From, _To>::value;
+#endif
 template<typename _Fn, typename... _Args>
   inline constexpr bool is_invocable_v = is_invocable<_Fn, _Args...>::value;
 template<typename _Fn, typename... _Args>