Swift: Control-flow for key paths

[MathiasVP]

In Swift one can define a kind of "first-class getter" called a key path expression. For example:

struct B {
  var x : Int
}

struct A {
  var bs : [B]
}

func giveMeTheFirstInt(a : A) -> Int {
  return a[keyPath: \A.bs[0].x]
}

As I understand it, the semantics of a[keyPath: \A.bs[0].x] is roughly the same as (fun arg:A -> arg.bs[0].x)(a), and that's also how I think we'll end up modeling this in dataflow: Each key path expression generates a callable whose body generates a bunch of read steps.

This PR makes each key path expression a new Scope in the CFG (and a Callable in dataflow). As can be seen in the tests we're still missing CFG for the body of the key path expressions: Most of the enter #keyPath(...) CFG nodes flows directly to a TBD (UnresolvedDotExpr).