Issue Description:

Swift really ought to have a way to specify exhaustive capture list. I really frequently want to specify an exhaustive list of captures, often empty.

Currently what I usually do (as an approximation) is to convert the closure to a static func with the 'captures' as parameters so I have reasonable confidence. Then I convert it back to a closure but this may break at any point in time.

Strawman syntax:

struct Foo {
   var bar: String = ""
   
   mutating func buz() {
       DispatchQueue(label: "foo").async { ![] in
           print("\(self.bar)") // compile error
       }
}

What the strawman syntax doesn't provide at the moment is a way to capture by reference (which is usually done for {{var}}s).