[libcxx] Add <experimental/chrono>

Summary:
This patch adds <experimental/chrono> which only contains a single variable template.

See: https://rawgit.com/cplusplus/fundamentals-ts/v1/fundamentals-ts.html#time

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

Reviewed By: mclow.lists

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D7352

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@227860 91177308-0d34-0410-b5e6-96231b3b80d8

include/experimental/__config

@@ -21,4 +21,8 @@
 #define _LIBCPP_END_NAMESPACE_LFTS  } } }
 #define _VSTD_LFTS _VSTD_EXPERIMENTAL::fundamentals_v1
 
+#define _LIBCPP_BEGIN_NAMESPACE_CHRONO_LFTS _LIBCPP_BEGIN_NAMESPACE_STD        \
+  namespace chrono { namespace experimental { inline namespace fundamentals_v1 {
+#define _LIBCPP_END_NAMESPACE_CHRONO_LFTS _LIBCPP_END_NAMESPACE_STD } } }
+
 #endif

include/experimental/chrono

@@ -0,0 +1,55 @@
+// -*- C++ -*-
+//===------------------------------ chrono ---------------------------------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef _LIBCPP_EXPERIMENTAL_CHRONO
+#define _LIBCPP_EXPERIMENTAL_CHRONO
+
+/**
+    experimental/chrono synopsis
+
+// C++1y
+
+#include <chrono>
+
+namespace std {
+namespace chrono {
+namespace experimental {
+inline namespace fundamentals_v1 {
+
+  // See C++14 20.12.4, customization traits
+  template <class Rep> constexpr bool treat_as_floating_point_v
+    = treat_as_floating_point<Rep>::value;
+
+} // namespace fundamentals_v1
+} // namespace experimental
+} // namespace chrono
+} // namespace std
+
+ */
+
+#include <experimental/__config>
+#include <chrono>
+
+#if _LIBCPP_STD_VER > 11
+
+_LIBCPP_BEGIN_NAMESPACE_CHRONO_LFTS
+
+#if __has_feature(cxx_variable_templates)
+
+template <class _Rep> _LIBCPP_CONSTEXPR bool treat_as_floating_point_v
+    = treat_as_floating_point<_Rep>::value;
+
+#endif /* __has_feature(cxx_variable_templates) */
+
+_LIBCPP_END_NAMESPACE_CHRONO_LFTS
+
+#endif /* _LIBCPP_STD_VER > 11 */
+
+#endif /* _LIBCPP_EXPERIMENTAL_CHRONO */