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

llvm-svn: 190395