Use a manipulator to add a value to the current diagnostic flag.

Summary:
This addresses the feedback to
http://lists.cs.uiuc.edu/pipermail/cfe-commits/Week-of-Mon-20140421/103598.html

Reviewers: rsmith

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D3453

llvm-svn: 206920