clang-format: Refactor calculation of lines intersecting with -lines.

No functional changes intended. However, it seems to have found a buggy
behavior in one of the tests. I think this structure is generally
desirable and it will make a planned bugfix significantly easier.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@195634 91177308-0d34-0410-b5e6-96231b3b80d8