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

[STL_MSFT]

Summary

[libcxx] [test] 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.

[STL_MSFT]

Thanks, r287382.

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

assert(globalMemCounter.checkOutstandingNewEq(0));
         std::function<int(int)> f(aref);
         assert(A::count == 1);
         assert(f.target<A>() == nullptr);
         assert(f.target<Ref>());
         std::function<int(int)> f2(std::move(f));
         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
         // is done without allocating
         DisableAllocationGuard guard;
         using Ptr = int(*)(int);
         Ptr p = g;
         std::function<int(int)> f(p);
         assert(f.target<A>() == nullptr);
         assert(f.target<Ptr>());
         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
 }