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

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@189582 91177308-0d34-0410-b5e6-96231b3b80d8