With this fix, Unity using IL2CPP should work with one of two
approaches:

- Call `FileDescriptor.ForceReflectionInitialization<T>` for every
  enum present in generated code (including oneof case enums)
- Ensure that IL2CPP uses the same code for int and any int-based
  enums

The former approach is likely to be simpler, unless IL2CPP changes
its default behavior. We *could* potentially generate the code
automatically, but that makes me slightly uncomfortable in terms of
generating code that's only relevant in one specific scenario. It
would be reasonably easy to write a tool (separate from protoc) to
generate the code required for any specific set of assemblies, so
that Unity users can include it in their application. We can always
decide to change to generate it automatically later.