Implements breaking string literals at slashes.

We now break at a slash if we do not find a space to break on.

Also fixes a bug where we would go over the limit when breaking the
second line.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@176350 91177308-0d34-0410-b5e6-96231b3b80d8