Support fieldless `recursively_pinned` structs and enums.
(In the case of structs, that's empty structs -- in the case of enums, it's C-like enums.)
PiperOrigin-RevId: 447449578
Extremely experimental interop tooling for C++ and Rust.
Please don‘t use, this is an experiment and we don’t yet know where will it take us. There will be breaking changes without warning. Unfortunately, we can't take contributions at this point.