[lld] Set up .arcconfig to point to new Diffusion LLD repository

Summary:
We want to automatically copy the appropriate mailing list
for review requests to the LLD 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
"LLD" for lld:

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

This explicitly updates lld's .arcconfig to point to the new C
repository in Diffusion, which will let us use Herald rule H270.

Reviewers: peter.smith, ruiu, rafael, sammccall

Reviewed By: sammccall

Subscribers: dlj, bkramer, sammccall, klimek

Differential Revision: https://reviews.llvm.org/D40495

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@319955 91177308-0d34-0410-b5e6-96231b3b80d8

.arcconfig

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