[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

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

@@ -81,4 +81,10 @@ int main()
     assert(globalMemCounter.checkOutstandingNewEq(0));
     assert(f.target<int (A::*)(int) const>() != 0);
     }
+    {
+      std::function<void(int)> f(&g);
+      assert(f);
+      assert(f.target<int(*)(int)>() != 0);
+      f(1);
+    }
 }

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

@@ -88,4 +88,11 @@ int main()
     assert(globalMemCounter.checkOutstandingNewEq(0));
     assert(f.target<int (A::*)(int) const>() != 0);
     }
+    {
+    std::function<void(int)> f;
+    f = &g;
+    assert(f);
+    assert(f.target<int(*)(int)>() != 0);
+    f(1);
+    }
 }

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

@@ -89,4 +89,12 @@ int main()
     fun(10);
     }
 #endif
+    {
+    std::function<void(int)> fun(std::allocator_arg,
+                                 test_allocator<int(*)(int)>(),
+                                 &g);
+    assert(fun);
+    assert(fun.target<int(*)(int)>() != 0);
+    fun(10);
+    }
 }