SingleWriterFixedHashMap: Check for __cpp_lib_atomic_is_always_lock_free
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

facebook/folly,facebook/folly,facebook/folly,facebook/folly,facebook/folly