shrink SingletonRelaxedCounter inline slow path
shrink SingletonRelaxedCounter inline slow path

Summary: Persuade the compiler to emit a tail call in the `SingletonRelaxedCounter` inline slow path.

Differential Revision: D27383131

fbshipit-source-id: 6f58b1eb23477c00221515d7d6e55a91668f7c84

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