Replace ObjRelocationInfo with relocation_iterator.

For MachO we need information that is not represented in ObjRelocationInfo.
Instead of copying the bits we think are needed from a relocation_iterator,
just pass the relocation_iterator down to the format specific functions.

No functionality change yet as we still drop the information once
processRelocationRef returns.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@180711 91177308-0d34-0410-b5e6-96231b3b80d8