We support three configurations: Xcode-only, CLT-only, and Xcode with
CLT. Our configuration output should correctly reflect this.
While MacOS::Xcode.version has to continue to return a guess if Xcode is
not installed in order to maintain backwards compatibility, this is an
implementation detail that we don't need to expose to the user. And it
makes `brew --config` output confusing.
So let's only print the "Xcode" line when an actual Xcode installation
is present. This makes it easy to quickly figure out which of the three
possible configurations the user is running.
Addresses Homebrew/homebrew#14941, more or less.
5ce1caa1f3