[clang-format] Support sorting using declarations

Summary:
This patch adds UsingDeclarationsSorter, a TokenAnalyzer that sorts consecutive
using declarations.

Reviewers: klimek

Reviewed By: klimek

Subscribers: Typz, djasper, cfe-commits, klimek, mgorny

Differential Revision: https://reviews.llvm.org/D33823

llvm-svn: 305901