Saves created temp directory if save-temps option is used.

[slo1]

Should fix #38068.

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @nikomatsakis (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 Due to the way GitHub handles out-of-date commits, this should also make it reasonably obvious what issues have or haven't been addressed.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.

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[shepmaster]

Thanks @slo1! We'll get @nikomatsakis or another highly qualified reviewer to check this out soon!

[Mark-Simulacrum]

Thanks!

@bors r+ rollup

[bors]

📌 Commit a5c29cb has been approved by Mark-Simulacrum

[bors]

⌛ Testing commit a5c29cb with merge c9bb935...

[bors]

☀️ Test successful - status-appveyor, status-travis
Approved by: Mark-Simulacrum
Pushing c9bb935 to master...