Rework the shadow struct that is layed out for Objective-C classes.

 - Superclasses are now always laid out their shadow structure at the
   first field.

 - Prior to this, the entire class heirarchy was flattened into a
   single structure which meant that alignment, padding, and bitfields
   weren't packed correctly (the ASTRecordLayout was correct however,
   which meant our debug info didn't coincide with ivar offsets, for
   example).

 - This is still very suboptimal, but I believe the ivar layout itself
   is now at least close to correct.

 - <rdar://problem/6773388> error: objc[29823]: layout bitmap sliding
   backwards

llvm-svn: 69771