Cleanup tests that fail in C++1z and with Clang 3.8

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@242581 91177308-0d34-0410-b5e6-96231b3b80d8

test/std/utilities/function.objects/func.wrap/func.wrap.func/func.wrap.func.con/F.pass.cpp

@@ -11,7 +11,7 @@
 
 // class function<R(ArgTypes...)>
 
-// function(nullptr_t);
+// function(F);
 
 #include <functional>
 #include <cassert>
@@ -87,4 +87,8 @@ int main()
       assert(f.target<int(*)(int)>() != 0);
       f(1);
     }
+    {
+        std::function <void()> f(static_cast<void (*)()>(0));
+        assert(!f);
+    }
 }