[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

test/libcxx/experimental/utilities/syserror/header.system_error.synop/includes.pass.cpp

@@ -0,0 +1,22 @@
+//===----------------------------------------------------------------------===//
+//
+//                     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.
+//
+//===----------------------------------------------------------------------===//
+
+// <experimental/system_error>
+
+#include <experimental/system_error>
+
+#ifndef _LIBCPP_STD_VER > 11
+# ifndef _LIBCPP_SYSTEM_ERROR
+#   error "<experimental/system_error> must include <system_error>"
+# endif
+#endif
+
+int main()
+{
+}

test/libcxx/experimental/utilities/syserror/version.pass.cpp

@@ -0,0 +1,20 @@
+//===----------------------------------------------------------------------===//
+//
+//                     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.
+//
+//===----------------------------------------------------------------------===//
+
+// <experimental/system_error>
+
+#include <experimental/system_error>
+
+#ifndef _LIBCPP_VERSION
+#error _LIBCPP_VERSION not defined
+#endif
+
+int main()
+{
+}