C#: Shared interface/implementation for flow summaries

[hvitved]

Replaces #4443.

https://jenkins.internal.semmle.com/job/Changes/job/CSharp-Differences/590/

[RasmusWL]

Very cool stuff 💪 I think it could be very interesting to adopt this for Python in the future 👍

From the Python side of things, we have the same problems that Max outlined for JS: we don't have a database entity representing the callable for external library functions (currently installation of external dependencies can fail, and moving forwards we're trying to avoid installing dependencies altogether). I'm not entirely sure what we would end up using for the language specific SummarizableCallable, will it be a problem if what we use is not a subclass of DataFlowCallable, but for example string?

@aschackmull also mentioned something about moving the compilation of delegates to the body of the callable instead of the call-site. I didn't fully understand this part, but wanted to hear if this is part of the C# specific implementation? -- I'm slightly worried about not having a real callable, but if it's intended to be kept as something C# specific in the long run, I guess there is no problem.

[hvitved]

From the Python side of things, we have the same problems that Max outlined for JS: we don't have a database entity representing the callable for external library functions (currently installation of external dependencies can fail, and moving forwards we're trying to avoid installing dependencies altogether). I'm not entirely sure what we would end up using for the language specific SummarizableCallable, will it be a problem if what we use is not a subclass of DataFlowCallable, but for example string?

You can choose whatever base class you prefer.

@aschackmull also mentioned something about moving the compilation of delegates to the body of the callable instead of the call-site. I didn't fully understand this part, but wanted to hear if this is part of the C# specific implementation? -- I'm slightly worried about not having a real callable, but if it's intended to be kept as something C# specific in the long run, I guess there is no problem.

For C# the calls have been moved inside the summarized callable. Even if you don't have an entity to represent a summarized callable, you can still synthesize one and add it as a branch of an IPA type that defines DataFlowCallable.

[hvitved]

I adjusted the compilation to include more sharing. Ready for review again.