Fix bug in LineState comparison function.

The key bug was

  if (Other.StartOfLineLevel < StartOfLineLevel) ..

instead of

  if (Other.StartOfLineLevel != StartOfLineLevel) ..

Also cleaned up the function to be more consistent in the comparisons.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@175500 91177308-0d34-0410-b5e6-96231b3b80d8