swift234

[ClangImporter] Add a mode where @class decls are imported as empty classes.

This can be used by SourceKit's interface printer to display modules with
forward declarations of classes in dependent modules, e.g. ObjectiveC
forward-declaring Foundation's NSString for use in NSObject. We can consider
doing something similar for C tag types (structs and enums).

Part of rdar://problem/18273845

Swift SVN r21828