Fix NullablePointer type constraint check being omitted in FFI declarations

[ergl]

We're only interested in ignoring the "guard" part of a use command, but
we still want to go through the arguments to check for any possible
errors during the expr pass. In particular, we're interested in the
NullablePointer constraint check.

---

043dbb1 added the original code that ignored use statements during the expr pass.

I don't know the specific rationale, but it seems we can do this check in a more fine-grained manner so that the rest of a use statement goes through the expr pass.

Fixes #3757

Edit: I made this a draft PR since I'm sure there could be a better way of doing this if I understood the reason for ignoring use statements.

[SeanTAllen]

I am going to defer to @jemc on this one. This isn't my areas of expertise.

[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 3758.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.

[jemc]

Good catch!

[SeanTAllen]

@ergl do you think this is deserving of a release ahead of schedule?

[ergl]

@SeanTAllen Although the original issue should only result in the compiler crashing, I had cases where compiling the offending code in debug mode produced a segfaulting binary, so I'd go with probably yes

[SeanTAllen]

@ergl ok. once this gets release notes, and is merged, i'll do a release this weekend.