Fix partial FFI declarations ignoring partial annotation

[ergl]

Previously, the compiler would ignore any ? annotations on FFI
declarations, which could result in the generated code lacking any
guards for errors, causing the final program to abort at runtime.

h/t to @jemc for noticing what needed to be fixed.

[SeanTAllen]

@ergl is this a change that impacts on end user code? should it have a changelog entry? I'm thinking no based on the description, but I'm not sure.

[SeanTAllen]

Actually thinking about it more, I'm going to add a CHANGELOG- fixed

[ergl]

@SeanTAllen I think this would only affect users that a) tried to raise an error from C code (doubtful) and b) used a declaration. My intuition is that it won't be common, but it could happen, I guess.

[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 3713.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]

This will go out in the next release which is currently scheduled around the end of the month.