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

[benhamilton]

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.

[klimek]

LG. Thx!

cfe/trunk/.arcconfig

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