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

[benhamilton]

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.

libcxx/trunk/.arcconfig

 {
-  "project_id" : "libcxx",
+  "repository.callsign" : "CXX",
   "conduit_uri" : "https://reviews.llvm.org/"
 }