[CodeCompletion] Map the result type for keypath member lookup

[rintaro]

struct Circle {
  var center: Point { get }
  var radius: Length { get }
}

@dynamicMemberLookup
struct Wrapper<T> {
  subscript<U>(dynamicMember: KeyPath<T, U>) -> Wrapped<U> { get }
}

func test(value: Wrapper<Circle>) {
  value.#HERE#
}

The type of center and radius should be Wrapped<Point> and Wrapped<Length> respectively.

rdar://problem/50073837

NOTE: This implementation doesn't properly handle:

• Nested wrapper
  (e.g. Wrapper<Wrapper<Point>>)
• Generic result type in KeyPath
  (e.g. subscript<U>(dynamicMember: KeyPath<T, Box<U>>) -> Wrapper<U>)

[rintaro]

@xedin, @DougGregor Could you check I'm doing right thing?

[rintaro]

@swift-ci Please smoke test

[xedin]

Seems reasonable to me, thanks Rintaro!