[Clang importer] Import Objective-C class properties as getters/setters. (#3449)
Objective-C introduced the notion of class properties post-Swift
2.2. To minimize disruption in Swift 2.2 source code as SDKs adopt
class properties in lieu of getters/setters, teach the Clang importer
to effectively ignore class properties. Fixes rdar://problem/25822005.