Swift ultimately requires a functional Swift toolchain to build the compiler. Even though there's a lot of the compiler still written in C++, it's a great milestone reached. But setting the minutiae of getting a bootstrap toolchain aside for a minute, in order to do this, you probably need to build swift-driver, which depends on Foundation, which depends on Dispatch.
Dispatch has a bunch of prs sitting that haven't been reviewed for a long while -- some that are necessary for portability changes, especially for new platform bringup. It's not really a great situation to have to write documentation that counsels or other build infrastructure that uses unsubmitted and unreviewed code to get Swift to build as part of regular process.
If I have not assessed the dependency problem correctly -- and in fact there's a way to sensibly get a useful Swift toolchain running without Dispatch -- do let me know. If Dispatch is truly at the bottom of the dependency tree, and it is nearly impossible to try and contribute meaningfully to Dispatch, then it certainly will remain the most frustrating aspect on trying to contribute to Swift. I hope that situation will change in the future.