useless_conversion: also fire inside compiler desugarings

[lapla-cogito]

Using e.span.from_expansion() alone prevented the lint from firing within compiler-desugared expressions, e.g. async fn, for loops, or ?.

changelog: [useless_conversion]: also fire inside compiler desugarings

close #16590

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[github-actions]

Lintcheck changes for d65fe0b

Lint	Added	Removed	Changed
clippy::useless_conversion	6	0	1

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