[libcxx] [test] D26815: Fix an assumption about the state of moved-from std::functions.

The Standard doesn't provide any guarantees beyond "valid but unspecified" for
moved-from std::functions. libcxx moves from small targets and leaves them
there, while MSVC's STL empties out the source. Mark these assertions as
libcxx-specific.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@287382 91177308-0d34-0410-b5e6-96231b3b80d8

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

@@ -132,7 +132,7 @@ int main()
         assert(A::count == 1);
         assert(f2.target<A>() == nullptr);
         assert(f2.target<Ref>());
-        assert(f.target<Ref>()); // f is unchanged because the target is small
+        LIBCPP_ASSERT(f.target<Ref>()); // f is unchanged because the target is small
     }
     {
         // Test that moving a function constructed from a function pointer
@@ -146,7 +146,7 @@ int main()
         std::function<int(int)> f2(std::move(f));
         assert(f2.target<A>() == nullptr);
         assert(f2.target<Ptr>());
-        assert(f.target<Ptr>()); // f is unchanged because the target is small
+        LIBCPP_ASSERT(f.target<Ptr>()); // f is unchanged because the target is small
     }
 #endif  // TEST_STD_VER >= 11
 }