nll: incorrect unused-mut warnings

[nikomatsakis]

There are a lot of incorrect unused_mut warnings coming up. I'm going to make this meta-bug for tracking them because it's very unclear how many underlying issues there are:

• [NLL] mutability difference #50897
• nll: incorrect unused_mut with move closure #51801
• [NLL] variable does not need to be mutable #51830
• NLL: incorrect "unused mut" lint warning, with multiple closures #51904

[matthewjasper]

There are at least two different issues here it seems:

fn foo(ref x: i32) {}

fn bar() {
    let mut x = 0;
    move || {
        x = 1;
    };
}

I'll try to have a PR for the first one over the weekend.
The second one requires move investigation, because the code is certainly trying to avoid linting in this case.