Don't trigger let_underscore_future when the type is specified

[mistaste]

Fixes #16991.

let_underscore_future fired even when the discarded value had an explicit type
annotation:

let _: Pin<Box<dyn Future<Output = ()>>> = Box::pin(some_async_fn());

Writing the type already shows intent, and the mem::drop suggestion is noisier
here. This makes let_underscore_future a strict subset of let_underscore_untyped
by only firing when no type is given (matching how let_underscore_untyped itself
is gated on local.ty.is_none()).

changelog: [let_underscore_future]: no longer fires when the let _ binding has an explicit type annotation

[rustbot]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @samueltardieu (or someone else) some time within the next two weeks.

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[rustbot]

☔ The latest upstream changes (possibly #17213) made this pull request unmergeable. Please resolve the merge conflicts.