Use isCharInSet from llvm/Support/UnicodeCharRanges.h, added a test for double-width characters in FixIt-hints.

Summary: This is a follow-up to r187837.

Reviewers: gribozavr, jordan_rose

Reviewed By: jordan_rose

CC: cfe-commits

Differential Revision: http://llvm-reviews.chandlerc.com/D1306