LLVM is written in C++ and I'm assuming that the Swift compiler developers are also very familiar with C++. It's also fairly straight forward to write binding for any language to things written in C/C++. The latter makes it at least theoretically possible to build tools on top of the compiler. C++ is really the obvious and simple choice, that you can get away with without large discussion.
Also if it's actually planned to make Swift self-hosting, the language you write the initial compiler in matters even less. If you're throwing it away anyway at the first opportunity, who cares about a few imperfections?
Which raises the question: Why was the compiler for Swift written in C++? I know writing a compiler in C/C++ is probably an instinctual reaction at this point, but um ... what's the advantage of C++? especially if the language will become self-hosting?