Fix crash when handling an argument parsing-related warning.

Summary: Don't crash on warnings coming before the translation unit starts.

Reviewers: klimek, djasper

Reviewed By: djasper

CC: cfe-commits

Differential Revision: http://llvm-reviews.chandlerc.com/D2676

git-svn-id: https://llvm.org/svn/llvm-project/clang-tools-extra/trunk@200702 91177308-0d34-0410-b5e6-96231b3b80d8