Add source URL to diagnostic about extra content in @Links directive

[d-ronnqvist]

Bug/issue #, if applicable: rdar://118478839

Summary

This fixes an issue where diagnostic about extra content in @Links directives didn't display on the line of that content because the diagnostic didn't have a source URL.

Dependencies

None

Testing

• In any project with either articles or documentation extension files, add an @Links directive with some extra content after one of the links, for example:

@Links(visualStyle: compactGrid) {
    - ``SomeSymbol`` Some extra content after the link
}

• Build documentation for that project. The "Extraneous content found after a link" warning should display the file and line that's causing the warning.

Checklist

Make sure you check off the following items. If they cannot be completed, provide a reason.

• Added tests
• Ran the ./bin/test script and it succeeded
• [ ] Updated documentation if necessary

[d-ronnqvist]

@swift-ci please test

[patshaughnessy]

FYI Chris Sanders just saw this in production and filed a new Radar for it.

[mayaepps]

Out of curiosity, is there a case where lowerBound.source may not exist but upperBound.source would exist?

[d-ronnqvist]

In theory it's possible but I don't think there's a real case where it would happen.. I believe in practice either one or both of the source locations would have a source.

[d-ronnqvist]

@swift-ci please test