Handle patterns within closures with the feature gate enabled

[arora-aman]

When capture_disjoint_fields is enabled we see that the compiler ICEs when closure contains something like

let x = 10;
let tup = (1, 2);
let p = Point { x: 10, y: 20 };

let c = || {
    let _ = x;
    let Point { x, y } = p; // 1
    let (_, _) = tup; // 2
};

The problem here is that in case 1 p[0] and p[1] are captured, but when we build MIR we need to get information about the initializer which is p in this case which is missing since p itself isn't captured.

The issue with 2 is that since nothing from tup is used, nothing is captured. Nothing will be read when MIR is built either, but to build MIR we need access to tup, which we don't have.

Ensure ui/closures/2229_closure_analysis/wild_patterns passes

[roxelo]

Once this is implemented, also modify the following test so the closure body is:

let SingleVariant::Point(ref mut x, _) = point;
*x += 1;

[arora-aman]

Iniital ideas: https://hackmd.io/fgc31ECfQnaxfP9DmjgVNw?view

[arora-aman]

More detailed ideas using fake reads for "upvars": https://hackmd.io/fgc31ECfQnaxfP9DmjgVNw