Use the same set of whitespace characters for all operations in BreakableToken.

Summary:
Fixes a problem where \t,\v or \f could lead to a crash when placed as
a first character in a line comment. The cause is that rtrim and ltrim handle
these characters, but our code didn't, so some invariants could be broken.

Reviewers: klimek

Reviewed By: klimek

CC: cfe-commits

Differential Revision: http://llvm-reviews.chandlerc.com/D1013