Improve unused variable report accuracy

[InKryption]

With unused variable warnings enabled, this code

const blk = blk: {
    break :blk;
};
blk: for ([_]void{}) |_| break :blk;
blk: while (false) break :blk;
_ = .blk;
_ = .{ .blk = .foo };

would have previously reported the blk variable as being used. With some extra checks for some surrounding tokens, blk should now be correctly reported as unused.

[SuperAuguste]

Fantastic, thanks!