Fix a crash when diagnostic points to a macro definition on command line.

Reviewers: klimek

Reviewed By: klimek

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D3620

git-svn-id: https://llvm.org/svn/llvm-project/clang-tools-extra/trunk@208068 91177308-0d34-0410-b5e6-96231b3b80d8