Ensure reachability of types returned by FFI calls

[ergl]

After introducing #4060, we forgot to re-add the code that ensured that, if a
FFI function call specified different return types, those types were being
marked as reachable.

This created a situation where programs that introduced a new type (for example,
a bare lambda) in the context of a FFI return type would crash the compiler.
This was due to having missing information about the type during code
generation.

[ponylang-main]

Hi @ergl,

The changelog - fixed label was added to this pull request; all PRs with a changelog label need to have release notes included as part of the PR. If you haven't added release notes already, please do.

Release notes are added by creating a uniquely named file in the .release-notes directory. We suggest you call the file 4149.md to match the number of this pull request.

The basic format of the release notes (using markdown) should be:

## Title

End user description of changes, why it's important,
problems it solves etc.

If a breaking change, make sure to include 1 or more
examples what code would look like prior to this change
and how to update it to work after this change.

Thanks.

[SeanTAllen]

Approved with nit about spacing in the additional.c

[ergl]

This is now ready to merge (I forgot one spacing change in my last revision). When this is merged, please use the description in the OP as the commit message.