fix SingletonThreadLocal::try_get
fix SingletonThreadLocal::try_get

Summary: Fixes an improper cast. `SingletonThreadLocal` allows a factory which returns any type, as long as that type can be reference-cast to the target type. So the new code must do a reference-cast.

Reviewed By: amlannayak

Differential Revision: D31022770

fbshipit-source-id: 0fce6008fc3e21b6082ec7dcf3c3f9019b541ae9

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