libstdc++: Add _GLIBCXX_DEBUG safe iterator C++20 iterator concept

	* include/debug/safe_iterator.h
	[__cpp_lib_concepts](_Safe_iterator<>::iterator_concept): Define for
	C++20.

From-SVN: r278717

libstdc++-v3/ChangeLog

+2019-11-26  François Dumont  <fdumont@gcc.gnu.org>
+
+	* include/debug/safe_iterator.h
+	[__cpp_lib_concepts](_Safe_iterator<>::iterator_concept): Define for
+	C++20.
+
 2019-11-25  Jonathan Wakely  <jwakely@redhat.com>
 
 	* include/bits/iterator_concepts.h (ranges::iter_swap): Fix parameter

libstdc++-v3/include/debug/safe_iterator.h

@@ -140,6 +140,10 @@ namespace __gnu_debug
       typedef typename _Traits::reference		reference;
       typedef typename _Traits::pointer			pointer;
 
+#if __cplusplus > 201703L && __cpp_lib_concepts
+      using iterator_concept = std::__detail::__iter_concept<_Iterator>;
+#endif
+
       /// @post the iterator is singular and unattached
       _Safe_iterator() _GLIBCXX_NOEXCEPT : _Iter_base() { }