swift234

Add the DynamicLookup protocol for lookup across all classes and protocols.

When performing member lookup into an existential that involves the
DynamicLookup protocol, look into all classes and protocols for that
member. References to anything found via this lookup mechanism are
returned as instances of Optional.

This introduces the basic lookup mechanics into the type
checker. There are still numerous issues to work through:
  - Subscripting isn't supported yet
  - There's no SILGen or IRGen support
  - The ASTs probably aren't good enough for the above anyway
  - References to generics will be broken
  - Ambiguity resolution or non-resolution

Thanks to Jordan for the patch wiring up DynamicLookup.


Swift SVN r7689