make conversion explicit in `findLastSet` for longer integral types
make conversion explicit in `findLastSet` for longer integral types

Summary:
When the param type is larger than `unsigned int`, the wider type `size{}` was
implicitly converted to the return type `unsigned int` in `findLastSet`. This
diff makes the conversion explicit.

Reviewed By: yfeldblum

Differential Revision: D8971589

fbshipit-source-id: 8828504c462e9296b84a746bcb7f701bc4a7d20e

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