Add replace self.init with Builtin.unreachable in Mirror

[jakedves]

Replaced self.init calls with Builtin.unreachable() calls that was previously not allowed due to a (now fixed) issue. If you could have a look and run the CI bot, that would be appreciated @AnthonyLatsis, thanks.

Just a note I think that this change resulted in more "unsupported" test cases which may be something to check.

Resolves #61326

[AnthonyLatsis]

Thanks, Jake. Did you run into a conflict? If so, could you rebase over an up-to-date main branch to avoid having this merge commit in the history?

Just a note I think that this change resulted in more "unsupported" test cases which may be something to check

Tests predefine the conditions in which they are skipped — an implementation detail is not expected to make a difference.

[jakedves]

Hey Anthony, there wasn't a merge conflict, just a message that my fork was N commits behind main, and so I pulled those commits in. I'm not familiar with rebasing, sorry.

[AnthonyLatsis]

there wasn't a merge conflict, just a message that my fork was N commits behind main

I see. Keep in mind that to test or merge a pull request, it shouldn’t matter how far behind your branch is as long as there are no conflicts with the target branch. Since there is no conflict, a rebase is not necessary. Could you simply hard-reset the merge commit?

[jakedves]

Ok I think that's been done

[AnthonyLatsis]

@swift-ci please smoke test

[Azoy]

This looks good to me, thanks!