Fix references in fragment rules

[msujew]

This is a fix for an issue exposed in #1638.

Essentially, this is caused by the combination of cross references, actions and fragment rule calls. The main culprit is that we temporarily create AST nodes for fragment rule calls (which wasn't really necessary in the first place) and then use those temporary elements for the Reference object. As these AST nodes never get into the final AST, they don't have a document reference and result in errors during the scoping phase.

This change simply removes the temporary AST element, and lets the parser assign the properties to the original AST node that called the fragment rule.

Testing this fix requires a more complicated testing setup, but hopefully the comment explains this well enough.

[sailingKieler]

Hi Mark,
I have a few remarks & questions, nothing spectacular, mainly to make a bit clearer what's going on.

I would appreciate to see one or two more test cases addressing potentially problematic cases with fragments+cross-references for the sake of documentation and future stability.

[msujew]

@sailingKieler Thanks for the review. I've added an additional test case for this.