swift234

Basic partial ordering for members of protocol extensions.

Implement simplistic partial ordering rules for members of protocol
extensions. Specifically:
  - A member of a concrete type is more specialized than a member of a
  protocol extension
  - A member of a protocol extension of P1 is more specialized than a
  member of a protocol extension of P2 if P1 inherits from P2

This achieves most of what rdar://problem/20335936 covers, but does
not yet handle ordering between constrained protocol extensions.

Swift SVN r26723