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