[openmp] Set up .arcconfig to point to new Diffusion OMP repository

[benhamilton]

Summary

We want to automatically copy the appropriate mailing list
for review requests to the openmp 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
"OMP" for OpenMP:

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

This explicitly updates openmp's .arcconfig to point to the new
OMP repository in Diffusion, which will let us use Herald rule H272
to automatically subscribe openmp-commits to review requests.

[grokos]

This seems to be in line with the new diffusion repo for CTE. Looks good.

.arcconfig

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