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

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.

Reviewers: hans, grokos, Hahnfeld

Reviewed By: grokos

Subscribers: sammccall, klimek, openmp-commits

Differential Revision: https://reviews.llvm.org/D40499

llvm-svn: 319254