[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/__functional_base

@@ -364,7 +364,7 @@ struct __weak_result_type<_Rp (_Cp::*)(_A1, _A2, _A3...) const volatile>
 
 template <class _Fp, class _A0, class ..._Args,
             class>
-inline _LIBCPP_INLINE_VISIBILITY
+inline _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX11
 auto
 __invoke(_Fp&& __f, _A0&& __a0, _Args&& ...__args)
     -> decltype((_VSTD::forward<_A0>(__a0).*__f)(_VSTD::forward<_Args>(__args)...))
@@ -374,7 +374,7 @@ __invoke(_Fp&& __f, _A0&& __a0, _Args&& ...__args)
 
 template <class _Fp, class _A0, class ..._Args,
             class>
-inline _LIBCPP_INLINE_VISIBILITY
+inline _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX11
 auto
 __invoke(_Fp&& __f, _A0&& __a0, _Args&& ...__args)
     -> decltype(((*_VSTD::forward<_A0>(__a0)).*__f)(_VSTD::forward<_Args>(__args)...))
@@ -386,7 +386,7 @@ __invoke(_Fp&& __f, _A0&& __a0, _Args&& ...__args)
 
 template <class _Fp, class _A0,
             class>
-inline _LIBCPP_INLINE_VISIBILITY
+inline _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX11
 auto
 __invoke(_Fp&& __f, _A0&& __a0)
     -> decltype(_VSTD::forward<_A0>(__a0).*__f)
@@ -396,7 +396,7 @@ __invoke(_Fp&& __f, _A0&& __a0)
 
 template <class _Fp, class _A0,
             class>
-inline _LIBCPP_INLINE_VISIBILITY
+inline _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX11
 auto
 __invoke(_Fp&& __f, _A0&& __a0)
     -> decltype((*_VSTD::forward<_A0>(__a0)).*__f)
@@ -407,7 +407,7 @@ __invoke(_Fp&& __f, _A0&& __a0)
 // bullet 5
 
 template <class _Fp, class ..._Args>
-inline _LIBCPP_INLINE_VISIBILITY
+inline _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX11
 auto
 __invoke(_Fp&& __f, _Args&& ...__args)
     -> decltype(_VSTD::forward<_Fp>(__f)(_VSTD::forward<_Args>(__args)...))