[Const Extraction] Resolve member references which are constant and embed their values

[dendiz]

Try to follow and resolve member references to their constant values and embed into the const values file. If they are not resolvable fall back to a MemberReference kind. This allows extraction for usages like

enum MyConstants {
  static let key = "mykey"
}

struct MyEntity: IndexedEntity {
   @Property(indexingKey: MyConstants.key) let someProperty: String
   ....
}

fixes rdar://184639902