[refactor] Move clang-rename into the clang repository

The core engine of clang-rename will be used for local and global renames in the
new refactoring engine, as mentioned in
http://lists.llvm.org/pipermail/cfe-dev/2017-June/054286.html.

The clang-rename tool is still supported but might get deprecated in the future.

Differential Revision: https://reviews.llvm.org/D34696

llvm-svn: 306840