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

[benhamilton]

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.

clang-tools-extra/trunk/.arcconfig

 {
-  "project_id" : "clang-tools-extra",
+  "repository.callsign" : "CTE",
   "conduit_uri" : "https://reviews.llvm.org/"
 }