New lint: wildcard pattern in let

I think there should be a warning about the wildcard pattern in let.
I can't imagine any situation where this makes sense, and there are cases where it's a mistake caused by taking _ for identifier though it's not.

Wrong:

{
    let _ = my scope guard or whatever
    // dropped right away

    do stuff
}

Correct:

{
    let _g = my scope guard or whatever

    do stuff

    // dropped at the end of the scope
}

Start from the issue's two Rust examples and investigate how Clippy defines and tests lints for suspicious let patterns. The change is complete when the wildcard pattern in the incorrect example produces a warning while the named guard in the correct example does not; no source file or test path is specified here.