Deprecate std::__is_nullptr_t type trait

This non-standard extension is redundant and unused by the library.

	* include/std/type_traits (__is_nullptr_t): Add deprecated attribute.

From-SVN: r274491

libstdc++-v3/ChangeLog

+2019-08-14  Jonathan Wakely  <jwakely@redhat.com>
+
+	* include/std/type_traits (__is_nullptr_t): Add deprecated attribute.
+
 2019-08-14  Edward Smith-Rowland  <3dw4rd@verizon.net>
 
 	Implement C++20 p0879 - Constexpr for swap and swap related functions.

libstdc++-v3/include/std/type_traits

@@ -597,11 +597,11 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     : public __is_null_pointer_helper<typename remove_cv<_Tp>::type>::type
     { };
 
-  /// __is_nullptr_t (extension).
+  /// __is_nullptr_t (deprecated extension).
   template<typename _Tp>
     struct __is_nullptr_t
     : public is_null_pointer<_Tp>
-    { };
+    { } _GLIBCXX_DEPRECATED;
 
   // Composite type categories.