Simplify LinkerScript::addOrphanSections. NFCI.

This patch moves a std::find to a new function. It also removes
the following piece of code. I believe it should be fine because all
tests still pass.

  unsigned Index = std::distance(Opt.Commands.begin(), I);
  assert(Sec->SectionIndex == INT_MAX || Sec->SectionIndex == Index);
  Sec->SectionIndex = Index;

llvm-svn: 315125