Recommit r275161 "[ELF] - Move section factory out from writer to make it reusable."

With fix:
* fixed compilation error under linux:

template <class ELFT> class OutputSectionFactory {
...
  typedef typename SectionKey<ELFT::Is64Bits> Key;

changed to:
template <class ELFT> class OutputSectionFactory {
...
  typedef typename elf::SectionKey<ELFT::Is64Bits> Key;

llvm-svn: 275166