[libcxxabi] Set up .arcconfig to point to new Diffusion CXXA repository

Summary:
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:

http://lists.llvm.org/pipermail/cfe-dev/2017-November/056032.html

Similar to D40500, I set up a new Diffusion repository with callsign
"CXXA" for libc++abi:

https://reviews.llvm.org/source/libcxxabi/

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

llvm-svn: 319713