[libcxx] Set up .arcconfig to point to new Diffusion CXX repository

Summary:
We want to automatically copy the appropriate mailing list
for review requests to the libc++ repository.

For context, see the proposal and discussion here:

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

Similar to D40179, I set up a new Diffusion repository with callsign
"CXX" for libc++:

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

This explicitly updates libcxx's .arcconfig to point to the new
CXX repository in Diffusion, which will let us use Herald rule H268.

Reviewers: halyavin, EricWF, mclow.lists, compnerd, mstorsjo

Reviewed By: compnerd

Subscribers: cfe-commits, bkramer, dlj, sammccall, klimek, smeenai, mstorsjo, krytarowski

Differential Revision: https://reviews.llvm.org/D40500

llvm-svn: 319666