Clear the diagnostic buffer after flushing it.

Reviewers: klimek

Reviewed By: klimek

CC: cfe-commits

Differential Revision: http://llvm-reviews.chandlerc.com/D2504

git-svn-id: https://llvm.org/svn/llvm-project/clang-tools-extra/trunk@198423 91177308-0d34-0410-b5e6-96231b3b80d8