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

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.

Reviewers: klimek, sammccall

Reviewed By: sammccall

Subscribers: llvm-commits, probinson, asb

Differential Revision: https://reviews.llvm.org/D41964

llvm-svn: 322374