Calculate the total length of a line up to each token up front.

This makes the tedious fitsIntoLimit() method unnecessary and I can
replace one hack (constructor initializers) by a slightly better hack.

Furthermore, this will enable calculating whether a certain part of a
line fits into the limit for future modifications.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@172604 91177308-0d34-0410-b5e6-96231b3b80d8