fix(needless_maybe_sized): don't lint in proc-macro-generated code

[ada4a]

Fixes #13360

changelog: [needless_maybe_sized]: don't lint in proc-macro-generated code

[rustbot]

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[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.

[ada4a]

@rustbot ready