Stop using internal AnalysisContextCollectionImpl

[scheglov]

According to #2566 the saving is less than 5%.
This is way less than I think is necessary to justify using internal APIs.

[srawlins]

I'd love for this API to be public.

[bwilkerson]

But it might justify enhancing the public API so that we can retain the performance improvement while still avoiding public API.

The need here is to disable the generation of warnings and lints. But another soon-to-be-needed control is to disable running plugins. I don't think we want to completely disable the use of analysis options, but I'm not sure what the right nobs are for controlling the analysis.