From Dave Lee's comment on my documentation PR (https://github.com/apple/swift/pull/33786#discussion_r483138703):
It would be ideal if an Xcode version check happened within scope of build-script, to fail early and report the issue in a clear way.
I've been bit before: using the wrong Xcode still kicks off a build, and then some point in the build, maybe a while later, a compilation error happens. The error is not likely to point to Xcode version as the root cause, which leads to searches for keywords found in the error message. If the person is lucky, someone else has already hit the error, posted on the forums, and received help telling them they need to use a newer version of Xcode.
It would be nice to have a check + error for this, which you could override with (say) a flag.