issue a diagnostic that says we couldn't parse it

[todo]

Presentations/2021/RoslynSourceCodeGenerators/code/SourceGenerators/SourceGeneratorSamples/SettingsXmlGenerator.cs

Lines 36 to 41 in 9be0387

	//TODO: issue a diagnostic that says we couldn't parse it
	return;
	}
	// create a class in the XmlSetting class that represnts this entry, and a static field that contains a singleton instance.

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This issue was generated by todo based on a TODO comment in 9be0387. It's been assigned to @ignatandrei because they committed the code.