It may not solve your immediate problem, but I would highly recommend setting up a parallel build of the compiler using ninja in the release configuration. This will allow you to use lit to run tests as described in the docs, but perhaps more importantly will run the tests ~10x faster than a debug build (20 min. vs 2.5 hours on my machine).
This thread also has a lot of really useful information about finding a build setup tailored to your needs, depending on which part of the project you'd like to work on.