We want to automatically copy the appropriate mailing list
for review requests to the libc++ repository.
For context, see the proposal and discussion here:
Similar to D40179, I set up a new Diffusion repository with callsign
"CXX" for libc++:
This explicitly updates libcxx's .arcconfig to point to the new
CXX repository in Diffusion, which will let us use Herald rule H268.