[libcxx] Add <experimental/system_error>

Summary:
This patch just adds the variable templates in <experimental/system_error>.

see: https://rawgit.com/cplusplus/fundamentals-ts/v1/fundamentals-ts.html#syserror


Reviewers: jroelofs, danalbert, K-ballo, mclow.lists

Reviewed By: mclow.lists

Subscribers: chandlerc, cfe-commits

Differential Revision: http://reviews.llvm.org/D7353

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@227973 91177308-0d34-0410-b5e6-96231b3b80d8

include/experimental/system_error

@@ -0,0 +1,59 @@
+// -*- C++ -*-
+//===-------------------------- system_error ------------------------------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is dual licensed under the MIT and the University of Illinois Open
+// Source Licenses. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef _LIBCPP_EXPERIMENTAL_SYSTEM_ERROR
+#define _LIBCPP_EXPERIMENTAL_SYSTEM_ERROR
+
+/**
+    experimental/system_error synopsis
+
+// C++1y
+
+#include <system_error>
+
+namespace std {
+namespace experimental {
+inline namespace fundamentals_v1 {
+
+  // See C++14 19.5, System error support
+  template <class T> constexpr bool is_error_code_enum_v
+    = is_error_code_enum<T>::value;
+  template <class T> constexpr bool is_error_condition_enum_v
+    = is_error_condition_enum<T>::value;
+
+} // namespace fundamentals_v1
+} // namespace experimental
+} // namespace std
+
+*/
+
+#include <experimental/__config>
+
+#if _LIBCPP_STD_VER > 11
+
+#include <system_error>
+
+_LIBCPP_BEGIN_NAMESPACE_LFTS
+
+#if __has_feature(cxx_variable_templates)
+
+template <class _Tp> _LIBCPP_CONSTEXPR bool is_error_code_enum_v
+    = is_error_code_enum<_Tp>::value;
+
+template <class _Tp> _LIBCPP_CONSTEXPR bool is_error_condition_enum_v
+    = is_error_condition_enum<_Tp>::value;
+
+#endif /* __has_feature(cxx_variable_templates) */
+
+_LIBCPP_END_NAMESPACE_LFTS
+
+#endif /* _LIBCPP_STD_VER > 11 */
+
+#endif /* _LIBCPP_EXPERIMENTAL_SYSTEM_ERROR */