You can fix it yourself, looks like there is no reaction from Apple for years anyway :(
$ sudo zsh -c 'grep -v "xcode.lang.comment.recursive" /Applications/Xcode.app/Contents/SharedFrameworks/SourceModel.framework/Versions/A/Resources/LanguageSpecifications/Swift.xclangspec > /Applications/Xcode.app/Contents/SharedFrameworks/DVTFoundation.framework/Versions/A/Resources/Swift.xclangspec'
After that, there are no errors and the highlight is back.
serge-iv
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Additional_Tools_for_Xcode_10.dmg
still have it, but 32bit version and looks like it very bugged on modern OS.