Homebrew's instructions to update Xcode CLT fail to address warning
https://github.com/Homebrew/brew/issues/5002


This bug report, which is closed as expected behaviour, has this solution:


  MikeMcQuaid: Run xcode-select --install. If that doesn't work check for Mojave system updates. If that doesn't work reboot your machine and try both again. If that doesn't work try to download and install the latest CLT from https://developer.apple.com/downloads/more/


From the discussion under it, it seems you need to have both Xcode's CLT and the /Library CLT.