swift234

Track parent typerefs for nominal and bound generic types

- Read the Parent pointer out of Class/Value metadata and create
  typerefs for them.
- Add Parent fields to NominalTypeRef and BoundGenericTypeRef.
- Add TypeRef::getSubstMap(), which creates a new generic argument
  map after substitution has taken place on it. This is used to
  continue to burrow into nested value types, where generic type
  parameters may have a different index.
- Use a DenseMap as that generic argument map.
- Unconditionally key the generic argument map with (Depth, Index)
- Clean up ordering and presentation of Index and Depth. In the rest
  of the compiler, Depth comes before Index.