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

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@206920 91177308-0d34-0410-b5e6-96231b3b80d8