The compiler build(s) requires at least 25 Gb of free disk space - I recommend double or even triple that if possible. Please ensure you have enough disk space re-run the build. It’s possible we can’t find XCTest because we ran out of disk space while writing out the modules.
To the earlier post:
One less drastic option to try is running build-script with --reconfigure to try to kick cmake’s caches. I doubt that’s going to have an impact in this case, since it seems like the script failed to build the clang module for XCTest.
For the LLVM issue, make sure you haven’t accidentally overwritten any files. You can use git reset in the directories update-checkout is complaining about.
For a per-target workflow, I wrote up some tips here. The relevant targets you need to build successfully are swift and swift-stdlib.