[ELF] Use the correct cast.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@172264 91177308-0d34-0410-b5e6-96231b3b80d8
[ELF] Add AtomLayout instead of using pair<Atom, pair<uint64_t, uint64_t>>.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@172265 91177308-0d34-0410-b5e6-96231b3b80d8
[ELF] Fix comment.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@172266 91177308-0d34-0410-b5e6-96231b3b80d8