Reapply r69771, with updates & fixes:

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

 - Superclasses are now always laid out in 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
   were incorrect (the ASTRecordLayout was correct however, which
   meant our debug info didn't coincide with ivar offsets, for
   example).

 - This is still very suboptimal (for example, ivar are looked up
   recursively, 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: 69811