Go: show FunctionModel steps in path summaries

[owen-mc]

It was highlighted in #13415 that we don't put steps from functions modeled using DataFlow::FunctionModel or TaintTracking::FunctionModel in path summaries, which can be confusing. This PR does that, showing both inputs and outputs. Here are screenshots showing the option of just inputs as well.

Inputs only:

Both inputs and outputs:

Note: as discussed in #13415, step 3 in the second picture goes back to the definition of the variable being tainted, in a way which is quite counter-intuitive. This is a consequence of our use of def-use flow.

The downside of doing both is that any path which goes through the definition of sample (step 3 in the second picture) will have it in the path explanation, even if the path doesn't go through the function that has been modeled. This should be fixed by #13473.

[owen-mc]

Note: this is even better when combined with #13473. In that case we'd want to show the input and output nodes.

[owen-mc]

I've dropped the commit which made it only do inputs. I think inputs and outputs is definitely better given that #13473 should make the outputs more sensible.

None of the test changes include changes to the select results, just nodes and edges, as you'd expect. I checked through 3 or 4 of them in detail and they were all good.

[mbg]

Looks good, subject to DCA results.

[owen-mc]

Nothing showed up in the performance evaluation so I'm merging.