[libc++] Add missing check for C++17 in test

This was never caught on our bots because this specific test is marked
as `XFAIL: !is-lockfree-runtime-function`, which is the case on our bots.

Author: ldionne

libcxx/test/std/atomics/atomics.types.operations/atomics.types.operations.req/atomic_is_lock_free.pass.cpp

@@ -30,8 +30,10 @@ struct TestFn {
 
     A a(t);
     bool b1 = std::atomic_is_lock_free(static_cast<const A*>(&a));
+#if TEST_STD_VER >= 17
     if (A::is_always_lock_free)
       assert(b1);
+#endif
 
     volatile A va(t);
     bool b2 = std::atomic_is_lock_free(static_cast<const volatile A*>(&va));