C#: Make the flow summary filtering in the adapter.

[michaelnebel]

It turns out that the current logic for dataflow applied to summarized callables is inconsistent.
The following can happen:
If a callable in source is also considered a summarized callable because there exist a summary of a super implementation that apply to all overrides, then the source callable will not be included in DataFlowSummarizedCallable.
However, the callable will get summary nodes for the parameters and return, which will cause the uniqueEnclosingCallable test to fail (as there doesn't exist a corresponding DataFlowSummarizedCallable for the parameter and return summary nodes).
Instead of aligning the summary node and dataflow summarised callable implementation, it seems more consistent (and to ensure code cohesion) to move the exclusion logic to the adapter.

Another solution could be to add branch condition for summary nodes

TFlowSummaryNode(FlowSummaryImpl::Private::SummaryNode sn) {
  exists(DataFlowCallable dfc | dfc.asSummarizedCallable() = sn.getSummarizedCallable())
} or

[github-actions]

⚠️ The head of this PR and the base branch were compared for differences in the framework coverage reports. A recent commit removed the previously reported differences.

[michaelnebel]

DCA looks good.

[hvitved]

Much nicer!