libstdc++: Optimize is_unbounded_array trait performance

This patch optimizes the performance of the is_unbounded_array trait by
dispatching to the new __is_unbounded_array built-in trait.

libstdc++-v3/ChangeLog:

	* include/std/type_traits (is_unbounded_array_v): Use
	__is_unbounded_array built-in trait.

Signed-off-by: Ken Matsui <kmatsui@gcc.gnu.org>

libstdc++-v3/include/std/type_traits

@@ -3533,11 +3533,16 @@ template<typename _Ret, typename _Fn, typename... _Args>
   /// True for a type that is an array of unknown bound.
   /// @ingroup variable_templates
   /// @since C++20
+# if _GLIBCXX_USE_BUILTIN_TRAIT(__is_unbounded_array)
+  template<typename _Tp>
+    inline constexpr bool is_unbounded_array_v = __is_unbounded_array(_Tp);
+# else
   template<typename _Tp>
     inline constexpr bool is_unbounded_array_v = false;
 
   template<typename _Tp>
     inline constexpr bool is_unbounded_array_v<_Tp[]> = true;
+# endif
 
   /// True for a type that is an array of known bound.
   /// @since C++20