lint: _-prefixed variables don't get an unused-mut warning.

Bringing it into line with the unused-variable one,

    fn main() {
        let mut _a = 1;
    }

will not warn that `_a` is never used mutably.

Fixes #6911.
auto merge of #10518 : huonw/rust/6911, r=alexcrichton

Bringing it into line with the unused-variable one,

    fn main() {
        let mut _a = 1;
    }

will not warn that `_a` is never used mutably.

Fixes #6911.