libstdc++: use new built-in trait __is_reference for std::is_reference

This patch gets std::is_reference to dispatch to new built-in trait
__is_reference.

libstdc++-v3/ChangeLog:

	* include/std/type_traits (is_reference): Use __is_reference built-in
	trait.

Signed-off-by: Ken Matsui <kmatsui@cs.washington.edu>

libstdc++-v3/include/std/type_traits

@@ -639,6 +639,12 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
   // Composite type categories.
 
   /// is_reference
+#if __has_builtin(__is_reference)
+  template<typename _Tp>
+    struct is_reference
+    : public __bool_constant<__is_reference(_Tp)>
+    { };
+#else
   template<typename _Tp>
     struct is_reference
     : public false_type
@@ -653,6 +659,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     struct is_reference<_Tp&&>
     : public true_type
     { };
+#endif
 
   /// is_arithmetic
   template<typename _Tp>