[libcxx] Fix PR 22468 - std::function<void()> does not accept non-void-returning functions

Summary:
The bug can be found here: http://llvm.org/bugs/show_bug.cgi?id=22468

`__invoke_void_return_wrapper` is needed to properly handle calling a function that returns a value but where the std::function return type is void. Without this '-Wsystem-headers' will cause `function::operator()(...)` to not compile. 

Reviewers: eugenis, K-ballo, mclow.lists

Reviewed By: mclow.lists

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D7444

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@228705 91177308-0d34-0410-b5e6-96231b3b80d8

include/functional

@@ -1367,7 +1367,8 @@ template<class _Fp, class _Alloc, class _Rp, class ..._ArgTypes>
 _Rp
 __func<_Fp, _Alloc, _Rp(_ArgTypes...)>::operator()(_ArgTypes&& ... __arg)
 {
-    return __invoke(__f_.first(), _VSTD::forward<_ArgTypes>(__arg)...);
+    typedef __invoke_void_return_wrapper<_Rp> _Invoker;
+    return _Invoker::__call(__f_.first(), _VSTD::forward<_ArgTypes>(__arg)...);
 }
 
 #ifndef _LIBCPP_NO_RTTI
@@ -1429,7 +1430,7 @@ class _LIBCPP_TYPE_VIS_ONLY function<_Rp(_ArgTypes...)>
     template <class _Fp>
         struct __callable<_Fp, true>
         {
-            static const bool value =
+            static const bool value = is_same<void, _Rp>::value ||
                 is_convertible<typename __invoke_of<_Fp&, _ArgTypes...>::type,
                                _Rp>::value;
         };