Issue Description:

The initial implementation of _bridgeAnythingToObjectiveC is valiantly written in Swift, but the Swift source-level language does not really expose the low-level manipulation primitives for type metadata, existentials, or optionals necessary to do a good job here, so it ends up leaning on a bunch of new runtime and standard library SPI. It would be cleaner from an ABI perspective, and likely better performing, to write this function purely in C++ using the low-level runtime APIs.