TokenKinds: _Atomic isn't a keyword in MS mode

Summary:
This is needed to allow MSVC's <atomic> header to properly parse.
It uses _Atomic as a class-id.

Reviewers: rnk

Reviewed By: rnk

CC: cfe-commits

Differential Revision: http://llvm-reviews.chandlerc.com/D2948

llvm-svn: 202901