isCharInSet refactoring.

Summary:
Made UnicodeCharSet a class, perform validity checking inside its
constructor instead of each isCharInSet call, use std::binary_search instead of
own implementation.
This patch comes with a necessary change in clang (sent separately).

Reviewers: jordan_rose, klimek

Reviewed By: klimek

CC: cfe-commits, rsmith

Differential Revision: http://llvm-reviews.chandlerc.com/D1534
Use new UnicodeCharSet interface.

Summary: This is a Clang part of http://llvm-reviews.chandlerc.com/D1534

Reviewers: jordan_rose, klimek, rsmith

Reviewed By: rsmith

CC: cfe-commits

Differential Revision: http://llvm-reviews.chandlerc.com/D1535