[let_and_return]: avoid linting when code between last stmt and return expr is cfg'd out

[y21]

Fixes #9150

This moves span_contains_cfg to utils and starts using it in let_and_return as well.

changelog: [let_and_return]: avoid linting when code between the last statement and the final return expression is #[cfg]ed out

[rustbot]

r? @xFrednet

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[xFrednet]

LGTM, thank you!

At one point, I would like some better support from rustc for stuff like this, but I'm also not quite sure how to do it better. For now, this is a solid solution.

[xFrednet]

@bors r+

[bors]

📌 Commit 9e82ad8 has been approved by xFrednet

It is now in the queue for this repository.

[bors]

⌛ Testing commit 9e82ad8 with merge 4ef57d3...

[bors]

☀️ Test successful - checks-action_dev_test, checks-action_remark_test, checks-action_test
Approved by: xFrednet
Pushing 4ef57d3 to master...