Suggest local variables for captured format args

[JohnTitor]

Fixes #114108

[rustbot]

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[rustbot]

This PR was rebased onto a different main commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.

[BoxyUwU]

Judging by the test changes this doesn't feel like a big diagnostic improvement. The suggested code changes are often larger/more verbose than before and if I was reviewing code where somebody followed those suggestions I would tell them to rewrite it.

Adding a note that says "consider moving this expression to a local variable and then using the local here instead" would be relatively small and teach people how things work without it being the main suggestion of the diagnostic. I think I would prefer that

[BoxyUwU]

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