I also usually have trouble getting --xcode to work properly. Here's the command that I'm currently using successfully (although my current checkout is from Dec 18):
utils/build-script --skip-build-benchmarks \
--skip-ios --skip-watchos --skip-tvos --swift-darwin-supported-archs "$(uname -m)" \
--sccache --debug --swift-disable-dead-stripping --test \
--xcode --skip-test-cmark --libswift=off
In this case I think you need the --skip-test-cmark flag. You may also need --libswift=off (can't remember why).
If you run into this issue, I've been able to fix it by commenting out this block in stdlib/public/Concurrency/CMakeLists.txt.