Xcode fails to generate source files from intent definition files when using the Legacy Build System

The workaround for this problem is the following: First, add a Run Script phase before the Compile Sources phase of your target:

xcrun intentbuilderc generate -input ${SRCROOT}/PATH/TO/Intents.intentdefinition -output ${SRCROOT}/Intents -classPrefix "" -language Swift -swiftVersion 5.0

Then, add all of the generated files from the output path specified in the command above to all required targets in your project.

Source: https://developer.apple.com/documentation/xcode_release_notes/xcode_11_4_release_notes