Fix assertion when breaking string literals with tab characters.

Summary: Fixes http://llvm.org/PR19368

Reviewers: djasper, klimek

Reviewed By: klimek

CC: cfe-commits, klimek

Differential Revision: http://reviews.llvm.org/D3379

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@206295 91177308-0d34-0410-b5e6-96231b3b80d8