MachO: Move LayoutPass to MachO directory.

The real user of the LayoutPass is now only Mach-O, so move that
pass out of the common directory to Mach-O directory.

"Core" architecture were using the LayoutPass. I modified that
to use a simple OrderPass. I think no one actually have authority
what feature should be in Core and what's not, but I believe the
LayoutPass is not very suitable for Core. Before more code starts
depending on the complex pass, it's better to remove that from
Core.

I could have simplified that pass because Mach-O is the only user
of the LayoutPass. For example, the second parameter of the
LayoutPass constructor can be converted from optional to mandatory.
I didn't do that in this patch to keep it simple. I'll do in a
followup patch.

http://reviews.llvm.org/D7311

llvm-svn: 228341