Fix false positive for unit_arg lint
Fixes#6447
To avoid false positives don't complain about unit args when they come from a path expression, e.g. a local variable.
**Note:** This is my first contribution to Clippy, so I might have messed up somewhere. Any feedback is welcome and I'm happy to work out any kinks.
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changelog: Do not lint unit arguments when they come from a path expression.
d5223be2e3