[libc++][NFC] Move monostate to its own header.

[Mordante]

Summary

The format library uses std::monostate, but not a std::variant.
Moving std::monostate to its own header allows the format library to
reduce the amount of included code.

[Mordante]

Add the proper C++ version guards.

libcxx/include/CMakeLists.txt

set(files
   __utility/in_place.h
   __utility/integer_sequence.h
   __utility/move.h
   __utility/pair.h
   __utility/piecewise_construct.h
   __utility/rel_ops.h
   __utility/swap.h
   __utility/to_underlying.h
+  __variant/monostate.h
   algorithm
   any
   array
   atomic
   barrier
   bit
   bitset
   cassert

libcxx/include/__variant/monostate.h

+// -*- 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___VARIANT_MONOSTATE_H
+#define _LIBCPP___VARIANT_MONOSTATE_H
+
+#include <__config>
+#include <__functional/hash.h>
+#include <cstddef>
+
+#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
+#pragma GCC system_header
+#endif
+
+_LIBCPP_PUSH_MACROS
+#include <__undef_macros>
+
+_LIBCPP_BEGIN_NAMESPACE_STD
+
+struct _LIBCPP_TEMPLATE_VIS monostate {};
+
+inline _LIBCPP_INLINE_VISIBILITY
+constexpr bool operator<(monostate, monostate) noexcept { return false; }
+
+inline _LIBCPP_INLINE_VISIBILITY
+constexpr bool operator>(monostate, monostate) noexcept { return false; }
+
+inline _LIBCPP_INLINE_VISIBILITY
+constexpr bool operator<=(monostate, monostate) noexcept { return true; }
+
+inline _LIBCPP_INLINE_VISIBILITY
+constexpr bool operator>=(monostate, monostate) noexcept { return true; }
+
+inline _LIBCPP_INLINE_VISIBILITY
+constexpr bool operator==(monostate, monostate) noexcept { return true; }
+
+inline _LIBCPP_INLINE_VISIBILITY
+constexpr bool operator!=(monostate, monostate) noexcept { return false; }
+
+template <>
+struct _LIBCPP_TEMPLATE_VIS hash<monostate> {
+  using argument_type = monostate;
+  using result_type = size_t;
+
+  inline _LIBCPP_INLINE_VISIBILITY
+  result_type operator()(const argument_type&) const _NOEXCEPT {
+    return 66740831; // return a fundamentally attractive random value.
+  }
+};
+
+_LIBCPP_END_NAMESPACE_STD
+
+_LIBCPP_POP_MACROS
+
+#endif // _LIBCPP___VARIANT_MONOSTATE_H

libcxx/include/module.modulemap

module std [system] {
   module valarray {
     header "valarray"
     export initializer_list
     export *
   }
   module variant {
     header "variant"
     export *
+
+    module __variant {
+      module monostate { header "__variant/monostate.h" }
+    }
   }
   module vector {
     header "vector"
     export initializer_list
     export *
   }
   module version {
     header "version"

libcxx/include/variant

 
 } // namespace std
 
 */
 
 #include <__availability>
 #include <__config>
 #include <__utility/forward.h>
+#include <__variant/monostate.h>
 #include <__tuple>
 #include <array>
 #include <compare>
 #include <exception>
 #include <functional>
 #include <initializer_list>
 #include <limits>
 #include <new>
inline _LIBCPP_INLINE_VISIBILITY
     visit(_Visitor&& __visitor, _Vs&&... __vs) {
   using __variant_detail::__visitation::__variant;
   _VSTD::__throw_if_valueless(_VSTD::forward<_Vs>(__vs)...);
   return __variant::__visit_value<_Rp>(_VSTD::forward<_Visitor>(__visitor),
                                        _VSTD::forward<_Vs>(__vs)...);
 }
 #endif
 
-struct _LIBCPP_TEMPLATE_VIS monostate {};
-
-inline _LIBCPP_INLINE_VISIBILITY
-constexpr bool operator<(monostate, monostate) noexcept { return false; }
-
-inline _LIBCPP_INLINE_VISIBILITY
-constexpr bool operator>(monostate, monostate) noexcept { return false; }
-
-inline _LIBCPP_INLINE_VISIBILITY
-constexpr bool operator<=(monostate, monostate) noexcept { return true; }
-
-inline _LIBCPP_INLINE_VISIBILITY
-constexpr bool operator>=(monostate, monostate) noexcept { return true; }
-
-inline _LIBCPP_INLINE_VISIBILITY
-constexpr bool operator==(monostate, monostate) noexcept { return true; }
-
-inline _LIBCPP_INLINE_VISIBILITY
-constexpr bool operator!=(monostate, monostate) noexcept { return false; }
-
 template <class... _Types>
 inline _LIBCPP_INLINE_VISIBILITY
 auto swap(variant<_Types...>& __lhs,
           variant<_Types...>& __rhs) noexcept(noexcept(__lhs.swap(__rhs)))
     -> decltype(__lhs.swap(__rhs)) {
   __lhs.swap(__rhs);
 }
 
size_t __res =
                          typename __alt_type::__value_type>;
                        return hash<__value_type>{}(__alt.__value);
                      },
                      __v);
     return __hash_combine(__res, hash<size_t>{}(__v.index()));
   }
 };
 
-template <>
-struct _LIBCPP_TEMPLATE_VIS hash<monostate> {
-  using argument_type = monostate;
-  using result_type = size_t;
-
-  inline _LIBCPP_INLINE_VISIBILITY
-  result_type operator()(const argument_type&) const _NOEXCEPT {
-    return 66740831; // return a fundamentally attractive random value.
-  }
-};
-
 #endif // _LIBCPP_STD_VER > 14
 
 _LIBCPP_END_NAMESPACE_STD
 
 _LIBCPP_POP_MACROS
 
 #endif // _LIBCPP_VARIANT