[PECOFF] Extract atom definitions as Atoms.h so that we can use them in other files.

Extract atom definitions as Atoms.h so that we can use them in other files.
Also applied clang-format to Atoms.h.

Reviewers: shankarke

CC: llvm-commits

Differential Revision: http://llvm-reviews.chandlerc.com/D995

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@184124 91177308-0d34-0410-b5e6-96231b3b80d8