In the process of trying to figure out this error (Clang: error: unknown argument: '-fswift-async-fp=always') I noticed that CI doesn't appear to be doing any Xcode builds. Are we testing the --xcode flag anywhere? I'd like to compare to a known-good setup to see what might be causing the issue in the linked thread.
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I know is a work around, but commenting this command swift/CMakeLists.txt at 78809726080ce61a4f73c013d542c91d5da35f2e · apple/swift · GitHub should make you able to build with --xcode locally.
The issue on the thread is caused by a flag introduced on clang that is passed on build to compile Concurrency C/C++ code. This works without --Xcode because the version of clang selected is the one built from source with those changes applied. But for Xcode clang version selected is the pre-built one shipped with Xcode itself which still doesn't have this flag available. So once Xcode ships with the version of clang that supports this flag it will not be a problem. I've spent some time looking into this trying to see if there was a way to force Xcode build to link against built from source clang, but couldn't do it due limited knowledge in that area 
and the workaround was enough to unblock the local build for sometime.
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