Add makeSemiFuture declarations to helpers.h
Add makeSemiFuture declarations to helpers.h

Summary:
[Folly] Add `makeSemiFuture` declarations to `helpers.h`.

For consistency with the `makeFuture` declarations that are also there. Definitions for both are found in `Future-inl.h`.

Reviewed By: LeeHowes

Differential Revision: D6281826

fbshipit-source-id: 4b22dd9086d05dbdebba358c6f569a772017949a

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