Summary:
Adds swift extension file as a separate subspec, as it is not required for the objc projects.

Also this fixes the current issue where the flipper sample app fails to build on xcode 11, as it fails to link the Swift specific literals.

Fixes this too https://github.com/facebook/yoga/issues/565

Reviewed By: SidharthGuglani

Differential Revision: D18373993

fbshipit-source-id: 0c058886a837c7ceafcd0167f878b3e0f7763aa1