[ELF] Refactor File.h.

 - Split createAtom() in lib/ReaderWriter/ELF/File.h into small methods.
 - Added comments to code in other methods.

No functionality changes.

Reviewers: shankarke

CC: llvm-commits

Differential Revision: http://llvm-reviews.chandlerc.com/D921

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@183696 91177308-0d34-0410-b5e6-96231b3b80d8