According to this Clang: error: unknown argument: '-fswift-async-fp=always' - #18 by Buttaface, the tag swift-DEVELOPMENT-SNAPSHOT-2021-09-14-a is OK.
I can verify it is true.
Btw, I'm building on Apple silicon, using Xcode 13.1.
The full command is SKIP_XCODE_VERSION_CHECK=1 ./utils/build-script --xcode --debug --skip-build-benchmarks --swift-darwin-supported-archs "$(uname -m)" --sccache --skip-ios --skip-tvos --skip-watchos --libswift=off.