IRgen: Move BitField LValues to just hold a reference to the CGBitFieldInfo.

 - Unfortunately, this requires some horrible code in CGObjCMac which always
   allocats a CGBitFieldInfo because we don't currently build a proper layout
   for Objective-C classes. It needs to be cleaned up, but I don't want the
   bit-field cleanups to be blocked on that.

llvm-svn: 100474