Do not use invalid iterators to fix Windows build.

std::vector::insert invalidates all iterators, so it was not safe to do

  Script->Opt.Commands.insert(++I, Make(ElfSym::End1));
  Script->Opt.Commands.insert(++I, Make(ElfSym::End2));

because after the first line, `I` is no longer valid.

This patch rewrites fixes the issue. I belive the new code without
higher-order functions is a bit more readable than before.

llvm-svn: 312570