Auto merge of #5132 - Eh2406:less_clones, r=alexcrichton

don't clone a BacktrackFrame if we are not going to backtrack

I think this is a fix for #5130.

Currently the only recoverable `activate` error does not corrupt `cx`, so this is definitely ok. We should be careful as we add more recoverable `activate` errors that they don't corrupt `cx` to the point where the error messages are affected.