swift234

Implement protocol inheritance, e.g.,

  protocol Document { var title : String }
  protocol Versioning { func bumpVersion() }
  protocol VersionedDocument : Document, Versioning { }

This commit covers the basic functionality of protocol inheritance, including:
  - Parsing & AST representation
  - Conforming to a protocol also requires conforming to its inherited
  protocols
  - Member lookup into a protocol also looks into its inherited
  protocols (results are aggregated; there is no name hiding)
  - Teach ErasureExpr to maintain lvalueness, so we don't end up
  performing a silly load/erase/materialize dance when accessing
  members from an inherited protocol.




Swift SVN r1804