swift234

Make ObjC-ness implicit to ObjC-inherited classes.

Add 'isObjC' as a property of ValueDecl, and set it during type checking if a class is either explicitly annotated with an [objc] attribute or inherits from an isObjC class, or if a func is a method of an isObjC class. Tweak the ClangImporter and other places that summon magic ValueDecl nodes to set up the decls they synthesize as isObjC. Replace logic in typechecking and IRGen that branched on the 'isObjC' attribute to now branch on the 'isObjC' property of ValueDecls.

Swift SVN r4078