[polly] Set up .arcconfig to point to new Diffusion PLO repository

Summary: We want to automatically copy the appropriate mailing list
for review requests to the polly 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
"PLO" for polly:

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

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

llvm-svn: 319056