Lattner had been the director of the Developer Tools department, including Xcode and similar compiler efforts around Swift/LLVM. Chris joined Apple in 2005 due to his work on LLVM/Clang. His wife is the president of the LLVM Foundation. Coming as a surprise today is that he's leaving Apple and no longer the Swift Project Lead, per this mailing list post.
This is a big loss for Apple but will be interesting to see where he ends up next and what he's planning to tackle in the compiler space. More details when they become available.
Update: Tesla has announced Chris Lattner is joining the company as VP of Autopilot Software.