Swift 4 branch plans

Hi all,

We’re wrapping up Swift 4, and it’s past time for us to branch swift-corelibs-foundation for Swift 4 to match the rest of the projects. Here is my proposal:

Monday: stop auto-merging from master to Swift 4. master becomes Swift 4 update work.
After Monday: We are working on merging changes from macOS 10.13 / iOS 11.0 into swift-corelibs-foundation. That will come after the branch point, to reduce risk of breaking stuff for Swift 4.

Thanks,
- Tony