Apply libc++ fix for <ranges> not compiling at all

Merge commit 44cdca37c01a from llvm git (by Arthur O'Dwyer):

  [libc++] Define `namespace views` in its own detail header.

  Discovered in the comments on D118748: we would like this namespace
  to exist anytime Ranges exists, regardless of whether concepts syntax
  is supported. Also, we'd like to fully granularize the <ranges> header,
  which means not putting any loose declarations at the top level.

  Differential Revision: https://reviews.llvm.org/D118809

contrib/llvm-project/libcxx/include/__ranges/views.h

@@ -0,0 +1,35 @@
+// -*- C++ -*-
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef _LIBCPP___RANGES_VIEWS
+#define _LIBCPP___RANGES_VIEWS
+
+#include <__config>
+
+#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
+#  pragma GCC system_header
+#endif
+
+_LIBCPP_BEGIN_NAMESPACE_STD
+
+#if _LIBCPP_STD_VER > 17 && !defined(_LIBCPP_HAS_NO_INCOMPLETE_RANGES)
+
+namespace ranges {
+
+namespace views { }
+
+} // namespace ranges
+
+namespace views = ranges::views;
+
+#endif // _LIBCPP_STD_VER > 17 && !defined(_LIBCPP_HAS_NO_INCOMPLETE_RANGES)
+
+_LIBCPP_END_NAMESPACE_STD
+
+#endif // _LIBCPP___RANGES_VIEWS

contrib/llvm-project/libcxx/include/module.modulemap

@@ -817,6 +817,7 @@ module std [system] {
         export functional.__functional.perfect_forward
       }
       module view_interface         { private header "__ranges/view_interface.h" }
+      module views                  { private header "__ranges/views.h" }
     }
   }
   module ratio {

contrib/llvm-project/libcxx/include/ranges

@@ -223,6 +223,7 @@ namespace std::ranges {
 #include <__ranges/take_view.h>
 #include <__ranges/transform_view.h>
 #include <__ranges/view_interface.h>
+#include <__ranges/views.h>
 #include <compare>          // Required by the standard.
 #include <initializer_list> // Required by the standard.
 #include <iterator>         // Required by the standard.
@@ -233,14 +234,4 @@ namespace std::ranges {
 #pragma GCC system_header
 #endif
 
-#if !defined(_LIBCPP_HAS_NO_INCOMPLETE_RANGES)
-
-_LIBCPP_BEGIN_NAMESPACE_STD
-
-namespace views = ranges::views;
-
-_LIBCPP_END_NAMESPACE_STD
-
-#endif // !defined(_LIBCPP_HAS_NO_INCOMPLETE_RANGES)
-
 #endif // _LIBCPP_RANGES

lib/libc++/Makefile

@@ -670,6 +670,7 @@ RNG_HEADERS+=	subrange.h
 RNG_HEADERS+=	take_view.h
 RNG_HEADERS+=	transform_view.h
 RNG_HEADERS+=	view_interface.h
+RNG_HEADERS+=	views.h
 .for hdr in ${RNG_HEADERS}
 RNG+=		${HDRDIR}/__ranges/${hdr}
 .endfor