[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

git-svn-id: https://llvm.org/svn/llvm-project/libcxxabi/trunk@319713 91177308-0d34-0410-b5e6-96231b3b80d8

.arcconfig

@@ -1,4 +1,4 @@
 {
-  "project_id" : "libcxxabi",
+  "repository.callsign" : "CXXA",
   "conduit_uri" : "https://reviews.llvm.org/"
 }