Handle zero-width and double-width characters in string literals and comments.

Summary:
Count column width instead of the number of code points. This also
includes correct handling of tabs inside string literals and comments (with an
exception of multiline string literals/comments, where tabs are present before
the first escaped newline).

Reviewers: djasper, klimek

Reviewed By: klimek

CC: cfe-commits, klimek

Differential Revision: http://llvm-reviews.chandlerc.com/D1601

llvm-svn: 190052