swift234

Third part of the cast folding and type-casts re-factoring.  Addressing JoeG's comments.

This patch does the following:
- Improvements and correctness fixes for conversions of existential metatypes. They may succeed if you have a concrete or existential metatype that conforms to the protocol. Based on Joe's review of my previous patch.
- Removes special-cases for AnyObject. AnyObject is handled as any other class existential.
- Improves folding of conversions from class existential metatypes to concrete non-class metatypes
- Improves comments.
- Adds more tests to cover new test-cases.
- Adjusts a few existing tests.

Swift SVN r25690