Implemented macro referencing for save analysis

[DanielJCampbell]

r? @nrc

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @nrc (or someone else) soon.

If any changes to this PR are deemed necessary, please add them as extra commits. This ensures that the reviewer can see what has changed since they last reviewed the code. Due to the way GitHub handles out-of-date commits, this should also make it reasonably obvious what issues have or haven't been addressed. Large or tricky changes may require several passes of review and changes.

Please see the contribution instructions for more information.

[nrc]

Could you be more explicit about what "local spans" are? And maybe a better name for the field is imported_macro_spans

[DanielJCampbell]

Change made

[nrc]

Why do you need the Box here?

[DanielJCampbell]

Changes have been made. Hopefully that should be everything (let me know if you're happy with the use of span_bug for checking visit_mac)

[nrc]

@bors: r+

[bors]

📌 Commit 1d32641 has been approved by nrc

[bors]

⌛ Testing commit 1d32641 with merge a4a249f...

[bors]

☀️ Test successful - auto-linux-32-nopt-t, auto-linux-32-opt, auto-linux-64-debug-opt, auto-linux-64-nopt-t, auto-linux-64-opt, auto-linux-64-x-android-t, auto-linux-cross-opt, auto-linux-musl-64-opt, auto-mac-32-opt, auto-mac-64-nopt-t, auto-mac-64-opt, auto-mac-ios-opt, auto-win-gnu-32-nopt-t, auto-win-gnu-32-opt, auto-win-gnu-64-nopt-t, auto-win-gnu-64-opt, auto-win-msvc-32-opt, auto-win-msvc-64-opt