fix atomic_fetch_flip under MSVC (#1774)
fix atomic_fetch_flip under MSVC (#1774)

Summary:
Pull Request resolved: https://github.com/facebook/folly/pull/1774

There are static-asserts which are copy-pasted from similar functions but which are not applicable to this one.

Reviewed By: Orvid

Differential Revision: D36416227

fbshipit-source-id: 3c6b9e8429eb1a904c3449811850a98451c53785

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