[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

llvm-svn: 319052