Pack ivar offsets together.

Ivar offsets for synthesized ivars are wrong, which could end up with a large
number of dirty pages because of ivar fixups at runtime. When we pack all of the
synthesized ivars into the same section, it limits the number of dirty pages
created. Place them in the "__DATA,__objc_ivar" section.
<rdar://problem/9374905>

llvm-svn: 130870