Analyzer assemblies are still being loaded in devenv

Even though we're running analyzers and source generators out of proc now (and supporting reloading of them), it seems we're still loading generators in-process. Setting a few breakpoints revealed that we have at least two places we're doing it:

1. The various bits of the analyzer diagnostic services where we cache diagnostic descriptors.
2. CompletionProviders that are coming from analyzer references.

I had thought we had replaced the first entirely with things that were caching results out of proc at this point, but it seems we haven't. For the completion providers, we may still have to run them in-proc (since we don't do out-of-proc for completion), but we could have the OOP process report if there are any completion providers, and then only in that case actually load them in-proc.