swift234

API Notes: Fix several bugs that effect availability info propagation.

 - Merge availability info by merging CommonEntityInfo as part of ObjCContextInfo.
 - Don't try to add Absent nullability to property setters (since it cannot be
   represented by ObjCMethodInfo).
 - Factor out lookupObjCContext and make sure we always look up in both
   class and protocol.

This will be tested by API notes drop, which will get committed separately.

Swift SVN r22089