Support for double width characters.

Summary: Only works for UTF-8-encoded files.

Reviewers: djasper

Reviewed By: djasper

CC: cfe-commits, klimek

Differential Revision: http://llvm-reviews.chandlerc.com/D1311

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@187935 91177308-0d34-0410-b5e6-96231b3b80d8