Include argument labels in identifiers

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11 
protocol FooProtocol {
  func doAThing(using: String)
}

struct ConformsUsingClosure: FooProtocol {
  var doAThing: (String)->Void = { print($0) }
}

let instance = ConformsUsingClosure()
instance.doAThing("where did my label go?")

let erasedInstance: FooProtocol = instance
erasedInstance.doAThing(using: "Okay, now we get a label?")

Since closures don't support argument labels, we'd have to drop the "using:" label when invoking the function in a concrete context. I asked about adding support for this before, and the consensus was that dropping the argument labels was not acceptable (so it's not a technical limitation - it's just something we don't want to support until we can preserve the labels).

If we could instead write this:

struct ConformsUsingClosure: FooProtocol {
  var doAThing: (using: String)->Void = { print($0) }
}

Then we get a nice, consistent calling syntax, and there shouldn't be any objection to allowing the closure to witness the function requirement.

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