Originally this was handled in LookupCppImpl in the switch on the
CoreInterface, but in subsequent refactorings it was lost, and we now
assume we are always looking for a C++ witness and CHECK that the
interface is not `IntFitsIn`.
Refactor LookupCppImpl to have a single switch up front on the
CoreInterface enum, instead of multiple. It's a quick early out for
`IntFitsIn` and delegates work to helper functions specific to each
other CoreInterface value.