[ELF][Writer] Refactor Section to not have atoms. Move atoms into AtomSection.

The purpose of this change is to simplify creating non-atom sections.
Previously _contentType, _sectionKind and _order were used for multiple
purposes and collided in places. This moves all of the Atom specific logic down
into AtomSection and makes Section just have raw Elf_Shdr flags.

llvm-svn: 175207