Use a pointer as the relocation iterator.

Since the relocation iterator walks only the relocations in one section, we
can just use a pointer and avoid fetching information about the section at
every reference.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@180189 91177308-0d34-0410-b5e6-96231b3b80d8