Fix a memory leak in MachOObjectFile.

MachOObjectFile owns a MachOObj, but never frees it. Both MachOObjectFile
and MachOObj want to own the MemoryBuffer, though, so we have to be careful
and give them each one of their own.

Thanks to Greg Clayton, Eric Christopher and Michael Spencer for helping
figure out what's going wrong here.

rdar://12561773

llvm-svn: 168923