Fixes error when splitting block comments.

When trying to fall back to search from the end onwards, we
would still find leading whitespace if the leading whitespace
went on after the end of the line.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@182886 91177308-0d34-0410-b5e6-96231b3b80d8