swift234

Serialize @objc as a DeclAttribute

This allows us to consistently print the AST, no matter if it was just parsed
or deserialized.

Note that we still serialize the isObjC bit from Decl, because it can be set or
cleared by the typechecker.  It is possible to have isObjC=true when there is
not attribute (the ObjC'ness was inferred), and it is possible that
isObjC=false while there is an attribute (when the attribute does not pass the
semantic check).  While we can represent the former with an implicit attribute,
the latter is harder to represent (maybe with an invalid bit on the attribute?)


Swift SVN r15935