Add new unusable_matches_bindings lint

[GuillaumeGomez]

Re-opening of #11661 (original PR is still open but seem dead. Discussion on zulip agreed that I could cherry-pick the commits and make the last changes asked so here we go. :)

changelog: Add new unusable_matches_bindings lint

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[GuillaumeGomez]

Arf, you keep getting picked up. Setting someone else.

r? clippy

[Jarcho]

This would be better as a change to the unused_variables message rather than it's own lint with a second message about the same thing.

[GuillaumeGomez]

Good point.

[GuillaumeGomez]

This is actually already triggered by rustc's unused_variable so can be closed.