SingleWriterFixedHashMap: Check for __cpp_lib_atomic_is_always_lock_free

Summary: Add check for `__cpp_lib_atomic_is_always_lock_free` for using `is_always_lock_free`.

Reviewed By: davidtgoldblatt

Differential Revision: D18827013

fbshipit-source-id: 796d1ce3c279a807198897c092464dc3aff734e3

folly/experimental/SingleWriterFixedHashMap.h

@@ -50,7 +50,7 @@ namespace folly {
 ///
 template <typename Key, typename Value>
 class SingleWriterFixedHashMap {
-#if !FOLLY_MOBILE
+#if __cpp_lib_atomic_is_always_lock_free
   static_assert(
       std::atomic<Value>::is_always_lock_free,
       "This implementation depends on having fast atomic "