[XG] XamlG generates invalid #line paths

[simonrozsival]

The source generator generates invalid #line values. The path to the original file seems to be relative to the generated .cs file path (even when the file is not emitted to the disk). VS Code cannot show the red squigly line in the .xaml file and clicking on the error in the Problems tab navigates to a non-existent file:

[StephaneDelcroix]

also, #line numbers are wrong for expanded markup, there's an issue for that