[llvm] Set up .arcconfig to point to Diffusion L repository

[benhamilton]

Summary

Thanks to probinson for noticing this in his review of D41956.

Now that we have repository callsigns set in all the other
LLVM/Clang projects' .arcconfig files, we can set the top-level
LLVM .arcconfig repository callsign to "L".

This will correctly Cc: llvm-commits@ on all review requests
sent out from the LLVM repo directory, using Herald rule H270.

[sammccall]

Thanks for sorting all this out Ben, these little workflow things make a big difference (at least to me!)

llvm/trunk/.arcconfig

 {
+  "repository.callsign" : "L",
   "conduit_uri" : "https://reviews.llvm.org/"
 }