Calculate and store ColumnWidth instead of CodePointCount in FormatTokens.

Summary:
This fixes various issues with mixed tabs and spaces handling, e.g.
when realigning block comments.

Reviewers: klimek, djasper

Reviewed By: djasper

CC: cfe-commits

Differential Revision: http://llvm-reviews.chandlerc.com/D1608

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@190395 91177308-0d34-0410-b5e6-96231b3b80d8