[Cohost] Consider supporting workspace diagnostics

[davidwengier]

In the past Razor has never supported workspace diagnostics, or closed file diagnostics. I don't know if there was a good reason for this, but under cohosting and with FUSE, it seems to me like there certainly isn't one now.

There are two basic ways we can handle this:

• Razor can support workspace/diagnostic LSP requests
  • We would want to improve our diagnostics handling in this case, to take advantage of ResultId etc. to cache properly
• We could let Roslyn report diagnostics for Razor generated files when doing Full Solution Analysis
  • We'd need to modify Roslyn to hook up span mapping, or use the line directives to do this
  • This would make it optional, based on the anaylsis scope settings that Roslyn already has
  • This would only report C# errors, but given those are the only ones that can break the build, that might be okay