Let users specify use_frameworks! in their Podfile #4121
Description
As reported by @bdotdub in #4108 (comment), pods written in Swift require use_frameworks! in the user's Podfile. gRPC's podspec isn't currently compatible with that, IIRC because of OpenSSL.
- Make OpenSSL's podspec compatible with frameworks (we have to fix it anyway to solve Support bitcode #4081).
- Use a pod written in Swift in the SwiftSample, to prevent a regression.