Fix compilation errors with Clang

	* include/bits/range_access.h (ranges::disable_sized_range)
	(ranges::begin, ranges::end, ranges::cbegin, ranges::cend)
	(ranges::rbegin, ranges::rend, ranges::crbegin, ranges::crend)
	(ranges::size, ranges::empty, ranges::data, ranges::cdata)
	(ranges::range, ranges::sized_range, ranges::advance, ranges::distance)
	(ranges::next, ranges::prev): Guard with __cpp_lib_concepts.
	* include/bits/stl_iterator.h (disable_sized_sentinel): Likewise.

From-SVN: r277589

libstdc++-v3/ChangeLog

 2019-10-29  Jonathan Wakely  <jwakely@redhat.com>
 
+	* include/bits/range_access.h (ranges::disable_sized_range)
+	(ranges::begin, ranges::end, ranges::cbegin, ranges::cend)
+	(ranges::rbegin, ranges::rend, ranges::crbegin, ranges::crend)
+	(ranges::size, ranges::empty, ranges::data, ranges::cdata)
+	(ranges::range, ranges::sized_range, ranges::advance, ranges::distance)
+	(ranges::next, ranges::prev): Guard with __cpp_lib_concepts.
+	* include/bits/stl_iterator.h (disable_sized_sentinel): Likewise.
+
 	* include/bits/alloc_traits.h (__cpp_lib_constexpr_dynamic_alloc):
 	Define.
 	(allocator_traits::_S_construct, allocator_traits::_S_destroy)

libstdc++-v3/include/bits/range_access.h

@@ -349,6 +349,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     __adl_data(_Container& __cont) noexcept(noexcept(data(__cont)))
     { return data(__cont); }
 
+#ifdef __cpp_lib_concepts
 namespace ranges
 {
   template<typename>
@@ -1061,6 +1062,7 @@ namespace ranges
     }
 
 } // namespace ranges
+#endif // library concepts
 #endif // C++20
 _GLIBCXX_END_NAMESPACE_VERSION
 } // namespace

libstdc++-v3/include/bits/stl_iterator.h

@@ -442,7 +442,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     make_reverse_iterator(_Iterator __i)
     { return reverse_iterator<_Iterator>(__i); }
 
-#  if __cplusplus > 201703L
+#  if __cplusplus > 201703L && defined __cpp_lib_concepts
   template<typename _Iterator1, typename _Iterator2>
     requires (!sized_sentinel_for<_Iterator1, _Iterator2>)
     inline constexpr bool disable_sized_sentinel<reverse_iterator<_Iterator1>,