[cxx-interop] Explicitly require `CxxSequence`'s iterator to be `CxxIterator`
[cxx-interop] Explicitly require `CxxSequence`'s iterator to be `CxxIterator`

This is needed for automatic synthesis of conformances to `CxxSequence` protocol.
It also makes typechecker errors easier to understand when they happen.
apple/swift,JGiola/swift,apple/swift,roambotics/swift,benlangmuir/swift,JGiola/swift,glessard/swift,roambotics/swift,benlangmuir/swift,JGiola/swift,glessard/swift,glessard/swift,roambotics/swift,JGiola/swift,glessard/swift,apple/swift,roambotics/swift,JGiola/swift,apple/swift,benlangmuir/swift,glessard/swift,roambotics/swift,benlangmuir/swift,apple/swift,benlangmuir/swift,glessard/swift,JGiola/swift,apple/swift,roambotics/swift,benlangmuir/swift