Add new roslyn code analyzers to support optimal Span/Memory uses within upstream repos.

[ahsonkhan]

We have prototype analyzers within corefxlab atm which we can use as a starting point that encapsulates the tips from here: https://www.codemag.com/Article/1807051/Introducing-.NET-Core-2.1-Flagship-Types-Span-T-and-Memory-T

Once we add them to buildtools, upstream repos (like corefx/coreclr/etc.) will automatically throw warnings during build (which are generally treated as errors) and could even get suggested code fixes going forward.

dotnet/corefxlab#2206
dotnet/corefxlab#2236

ML.NET has their own internal code analyzers and we should consider how to support that (and potentially other) repos as well:
https://github.com/dotnet/machinelearning/tree/master/tools-local/Microsoft.ML.InternalCodeAnalyzer

cc @KrzysztofCwalina, @joshfree, @weshaggard, @eerhardt, @terrajobst

[eerhardt]

buildtools is going away. I think this issue should be opened in https://github.com/dotnet/arcade.

[ahsonkhan]

I talked to @weshaggard about this and he suggested keeping it in buildtools for the time being with the rest of the analyzers until we have a plan to move them all together.

[eerhardt]

It would seem that folks like the SDK team and ASP.NET would benefit from these analzyers as well. Getting them in arcade sooner would speed that up IMO.

Why put something here now, when you know it will be moved in less than a month or two?

[weshaggard]

@eerhardt it is just adding additional analyzers to our existing set so whether or not that it is done here or in arcade I don't think it makes much of a difference.

That said if @ahsonkhan is willing to do the work-item to move the analyzers to arcade as part of this work I'm all for it :)

[ViktorHofer]

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