[libcxx] Add <experimental/tuple> header for LFTS.

Summary:
This patch adds the `<experimental/tuple>` header (almost) as specified in the latest draft of the library fundamentals TS.

The main changes in this patch are:

1. Added variable template `tuple_size_v`
2. Added function `apply(Func &&, Tuple &&)`.
3. Changed `__invoke` to be `_LIBCPP_CONSTEXPR_AFTER_CXX11`.

The `apply(...)` implementation uses `__invoke` to invoke the given function. `__invoke` already provides the required functionality. Using `__invoke` also allows `apply` to be used on pointers to member function/objects as an extension. In order to facilitate this `__invoke` has to be marked `constexpr`. 



Test Plan:
Each new feature was tested. 

The test cases for `tuple_size_v` are as follows:
1. tuple_size_v.pass.cpp
  - Check `tuple_size_v` on cv qualified tuples, pairs and arrays.
2. tuple_size_v.fail.cpp
  - Test on reference type.
3. tuple_size_v_2.fail.cpp
  - Test on non-tuple
4. tuple_size_v_3.fail.cpp
  - Test on pointer type.

The test cases for tuple.apply are as follows:

1. arg_type.pass.cpp
   - Ensure that ref/pointer/cv qualified types are properly passed.
2. constexpr_types.pass.cpp
   - Ensure constexpr evaluation of apply is possible for `tuple` and `pair`.
3. extended_types.pass.cpp
   - Test apply on function types permitted by extension.
4. large_arity.pass.cpp
   - Test that apply can evaluated on tuples and arrays with large sizes.
5. ref_qualifiers.pass.cpp
   - Test that apply respects ref qualified functions.
6. return_type.pass.cpp
   - Test that apply returns the proper type.
7. types.pass.cpp
   - Test apply on function types as required by LFTS.

Reviewers: mclow.lists

Reviewed By: mclow.lists

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D4512

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@232515 91177308-0d34-0410-b5e6-96231b3b80d8

include/experimental/tuple

@@ -0,0 +1,81 @@
+// -*- C++ -*-
+//===----------------------------- tuple ----------------------------------===//
+//
+//                     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_TUPLE
+#define _LIBCPP_EXPERIMENTAL_TUPLE
+
+/*
+    experimental/tuple synopsis
+
+// C++1y
+
+#include <tuple>
+
+namespace std {
+namespace experimental {
+inline namespace fundamentals_v1 {
+
+  // See C++14 20.4.2.5, tuple helper classes
+  template <class T> constexpr size_t tuple_size_v
+    = tuple_size<T>::value;
+
+  // 3.2.2, Calling a function with a tuple of arguments
+  template <class F, class Tuple>
+  constexpr decltype(auto) apply(F&& f, Tuple&& t);
+
+} // namespace fundamentals_v1
+} // namespace experimental
+} // namespace std
+
+ */
+
+# include <experimental/__config>
+
+#if _LIBCPP_STD_VER > 11
+
+# include <tuple>
+# include <utility>
+# include <__functional_base>
+
+#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
+#pragma GCC system_header
+#endif
+
+_LIBCPP_BEGIN_NAMESPACE_LFTS
+
+#ifndef _LIBCPP_HAS_NO_VARIABLE_TEMPLATES
+template <class _Tp>
+_LIBCPP_CONSTEXPR size_t tuple_size_v = tuple_size<_Tp>::value;
+#endif
+
+template <class _Fn, class _Tuple, size_t ..._Id>
+inline _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX11
+decltype(auto) __apply_tuple_impl(_Fn && __f, _Tuple && __t,
+                                  integer_sequence<size_t, _Id...>) {
+    return _VSTD::__invoke(
+        _VSTD::forward<_Fn>(__f),
+        _VSTD::get<_Id>(_VSTD::forward<_Tuple>(__t))...
+    );
+}
+
+template <class _Fn, class _Tuple>
+inline _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX11
+decltype(auto) apply(_Fn && __f, _Tuple && __t) {
+    return _VSTD_LFTS::__apply_tuple_impl(
+        _VSTD::forward<_Fn>(__f), _VSTD::forward<_Tuple>(__t),
+        make_index_sequence<tuple_size<typename decay<_Tuple>::type>::value>()
+    );
+}
+
+_LIBCPP_END_NAMESPACE_LFTS
+
+#endif /* _LIBCPP_STD_VER > 11 */
+
+#endif /* _LIBCPP_EXPERIMENTAL_TUPLE */