or that interface with core and are only for internal use

- Make sure all C-core parameters are passed into C++ methods.
- Add Destroy() methods for ChannelData and CallData.
- Use C++-style casts.
- Add 'extern "C"' to iomgr/closure.h, which is used in C++ filters.

- When receiving a Trailers-Only response, return the metadata as
  trailing metadata instead of initial metadata.
- Send Trailers-Only response when we have no non-default initial metadata,
  no message to send, and trailing metadata to send.