Revert of r255626 "[ELF] - refactor of code in RelocationSection<ELFT>::writeTo()"

as it broke buildbot: 
http://lab.llvm.org:8011/builders/lld-x86_64-darwin13/builds/17836/steps/build_Lld/logs/stdio
/Users/buildslave/as-bldslv9/lld-x86_64-darwin13/llvm.src/tools/lld/ELF/OutputSections.cpp:268:14: error: redefinition of 'Rel'
    unsigned Rel;   ^
/Users/buildslave/as-bldslv9/lld-x86_64-darwin13/llvm.src/tools/lld/ELF/OutputSections.cpp:241:34: note: previous definition is here
  for (const DynamicReloc<ELFT> &Rel : Relocs) {

That compiles fine on my MSVS 2015 thought.

llvm-svn: 255628