Various IDE scenarios will stop working without document services

[davidwengier]

After #6919 is implemented we will no longer have any document services running in Roslyn, which means no span mapping service, which means, for example, when Roslyn performs a rename that touches a generated file, it won't call us to map the edits to the corresponding Razor file. This would also affect things like Go to Definition, Find All References, etc.

All of the information is available in the #line mappings in the generated file (especially if we enable enhanced line mappings) so potentially we could get Roslyn to host a span mapping service of a fashion, that is hooked up via the source generator, but that seems a lot like the document services approach that Roslyn wants to move away from.

Other ideas welcome.

See also #8671, though that only requires any kind of span mapping service to be present, rather than an actual working service.