…x tree for a C# document

This was a subtle future bug, that we would have hit when we removed the C# syntax tree bits in future. By checking for Razor GTD first we weren't deferring to Roslyn for plain attributes (ie, the case without `@bind-` in the tests) which means at best we do some extra unnecessary work, and at worst we could lose features for things Roslyn supports but we don't.

I did not intend to find this bug when I started. Sorry for missing it in the PR Dustin. The key is that `ignoreAttributes` should have been `true` when porting over the code, and changing that would have made the test fail in your branch.

See comment in the file, but essentially this is the only way for the code that returns the generated documents syntax tree to be hit in cohosting.

In light of "should we get rid of document context" convo this morning, figured this made sense to do (and I needed _something_ on IDocumentSnapshot in order to actually make this whole idea work)