I wonder what the plan is for getting swift interop with C++, given that all the Be/Haiku APIs are C++ (IIRC) and swift has no ability to call into C++ code directly.
But giving the way the Swift compiler is itself written in C++ and tied to the LLVM, i bet it will be "easy" for the Swift compiler team to create interoperability between C++ and Swift.
I think their current goal with ABI compability is one step in that direction.. so it will be just a question of Swift understanding C++ mangling in elf/coff/mach-o objects for the link time, and interpret C++ headers using Clang for the compile time.
Given all that, is not hard to predict that Swift will probably have the best interop with C++ code than any other language out there.