[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
Fixed a problem in the expression parser that
caused the IR interpreter not to work if the
process had finished running.

<rdar://problem/14124301>