Don't emit diagnostics for system headers.

Reviewers: djasper

Reviewed By: djasper

CC: cfe-commits

Differential Revision: http://llvm-reviews.chandlerc.com/D2519

git-svn-id: https://llvm.org/svn/llvm-project/clang-tools-extra/trunk@198767 91177308-0d34-0410-b5e6-96231b3b80d8