significant_drop_tightening: handle complex let-stmts

[ada4a]

Fixes #15602

This implements the "make the reconstruction more robust" approach, because the other one turned out to be somewhat complicated -- one would need to find the exact place the temporary was used in, and suggest replacing exactly that.

Based on #15598, but mostly for easier rebasing later.

changelog: [significant_drop_tightening]: handle complex let-statements

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[ada4a]

r? clippy

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[github-actions]

Lintcheck changes for 519964e

Lint	Added	Removed	Changed
clippy::significant_drop_tightening	0	0	54

This comment will be updated if you push new changes

[rustbot]

This PR was rebased onto a different master 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.

[rustbot]

☔ The latest upstream changes (possibly eee8ef8) made this pull request unmergeable. Please resolve the merge conflicts.

[llogiq]

I'm a bit unsure about those two suggestions. Is there a way we can do that without it becoming unrustfixable?