I was just able to build a Xcode build successfully as suspected with --debug (the whole command SKIP_XCODE_VERSION_CHECK=1 ./swift/utils/build-script --xcode --debug --skip-build-benchmarks --swift-darwin-supported-archs "$(uname -m)" --sccache --skip-ios --skip-tvos --skip-watchos --libswift=off)
Although I think it is an issue not be able to build with --release-debuginfo --debug-swift --debug works at a cost of some disk space and slower build time.
@mishal_shah Can I open an SR for that issue? Just asking before open because maybe this is already reported somewhere
Note that to build successfully, we need also to apply another workaround for unsupported flag: No Xcode builds on CI - #2 by LucianoPAlmeida