[PECOFF][Writer] Separate AtomChunk from SectionChunk.

The optional data directory header contains addresses to some atoms such
as the import address table in data section. Such fields can naturally
be set by relocation if we make the optional data driectory as an atom.

Currently we assume that atoms are always in a section, so we can't create
a file header with atoms. This patch separates section chunk from atom
chunk, to allow atom-based file header.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@185521 91177308-0d34-0410-b5e6-96231b3b80d8