[clang-tools-extra] Set up .arcconfig to point to new Diffusion CTE repository

Summary:
I'm testing out a new Diffusion repository `CTE`:

https://reviews.llvm.org/source/clang-tools-extra/

This explicitly updates clang-tools-extra's `.arcconfig` to point to
the new `CTE` repository in Diffusion, which will let us set up Herald
rules, etc.

Reviewers: klimek, sammccall

Reviewed By: sammccall

Subscribers: bkramer, dlj

Differential Revision: https://reviews.llvm.org/D40179

llvm-svn: 319039