[libcxxabi] Set up .arcconfig to point to new Diffusion CXXA repository
We want to automatically copy the appropriate mailing list
for review requests to the libc++abi repository.
For context, see the proposal and discussion here:
Similar to D40500, I set up a new Diffusion repository with callsign
"CXXA" for libc++abi:
This explicitly updates libcxxabi's .arcconfig to point to the new
CXX repository in Diffusion, which will let us use Herald rule H268.
Reviewers: phosek, beanz, EricWF, compnerd
Reviewed By: phosek
Subscribers: cfe-commits, klimek, sammccall, dlj, bkramer
Differential Revision: https://reviews.llvm.org/D40501