Updating llvm/clang stable branches

Hello swift-dev!

We are going to be performing an update to the llvm/clang stable branches within the next couple of days. I will send out an email before I pull the trigger. We will be using a -s ours merge so if you run update-checkout (or even just git pull), everything should fast forward appropriately.

If you have any questions, please feel free to send me an email (or respond to this one).