swift234

IRGen: Emit type references for remote reflection

- Implement emission of type references for nominal type field
  reflection, using a small custom encoder resulting in packed
  structs, not strings. This will let us embed 7-bit encoded
  32-bit relative offsets directly in the structure (not yet
  hooked in).
- Use the AST Mangler for encoding type references
  Archetypes and internal references were complicating this before, so we
  can take the opportunity to reuse this machinery and avoid unique code
  and new ABI.

Next up: Tests for reading the reflection sections and converting the
demangle tree into a tree of type references.

Todo: For concrete types, serialize the types for associated types of
their conformances to bootstrap the typeref substitution process.

rdar://problem/15617914