Implement the Objective-C #keyPath expression, which maps a sequence
of @objc property accesses to a key-path suitable for use with
Cocoa[Touch]. The implementation handles @objc properties of types
that are either @objc or can be bridged to Objective-C, including the
collections that work with key-value coding (Array/NSArray,
Dictionary/NSDictionary, Set/NSSet).
Still to come: code completion support and Fix-Its to migrate string
literal keypaths to #keyPath.
Implements the bulk of SR-1237 / rdar://problem/25710611.
