work around gcc < 7 in UniqueInstance
work around gcc < 7 in UniqueInstance

Summary: Some older versions of gcc cannot deduce the relevant types correctly, and they are still in use in corners of the codebase.

Differential Revision: D28111907

fbshipit-source-id: 0367cb102f450f0f601350308a5831c7fd68f775

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