[clang] Set up .arcconfig to point to new Diffusion C repository

Summary:
We want to automatically copy cfe-commits@ on review requests
to the clang repository.

Similar to D40179, I set up a new Diffusion repository with callsign
"C" for clang:

https://reviews.llvm.org/source/clang/

This explicitly updates clang's .arcconfig to point to the new C
repository in Diffusion, which will let us use Herald rule H268.

Reviewers: klimek, sammccall

Reviewed By: klimek

Subscribers: dlj, bkramer

Differential Revision: https://reviews.llvm.org/D40494

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@319052 91177308-0d34-0410-b5e6-96231b3b80d8

.arcconfig

@@ -1,4 +1,4 @@
 {
-  "project_id" : "clang",
+  "repository.callsign" : "C",
   "conduit_uri" : "https://reviews.llvm.org/"
 }