Implement fallback split point for string literals.

If we don't find a natural split point (currently space) in a string
literal protruding over the line, we just split at the last possible
point.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@176349 91177308-0d34-0410-b5e6-96231b3b80d8