C++: Add Parameter nodes for functions without bodies

[MathiasVP]

This PR fixes an annoyance with ParameterNodes in C/C++ dataflow: Parameters of functions without a definition (i.e., when we only have a set of declarations) don't generate ParameterNodes.

This is because we define ParameterNodes using the InitializeParameter instruction in the IR, and without a function body there's no instructions associated with the function.

This PR fixes this by generating new dataflow nodes (from Parameters) and marking those as ParameterNodes.

Ideally, only 98a3f2d and 06f52c2 would've been necessary. However, the current setup for marking parameter nodes wasn't very easy to extend to something that wasn't defined in terms of Instructions so it took some refactoring (see 111ad8b) to allow for this. That commit should be totally behavior preserving.

Commit-by-commit review recommended.

cc @bdrodes

[jketema]

I'm missing something - I think. How do we know that the bodiless parameter nodes are actually only for functions without bodies? I don't see an explicit condition for this anywhere.

[MathiasVP]

I'm missing something - I think. How do we know that the bodiless parameter nodes are actually only for functions without bodies? I don't see an explicit condition for this anywhere.

That's the not any(InitializeParameterInstruction init).getParameter() = p part that's in the IPA branch for the new dataflow nodes. At least, that's what I intended when I wrote it 😂

[jketema]

I'm missing something - I think. How do we know that the bodiless parameter nodes are actually only for functions without bodies? I don't see an explicit condition for this anywhere.

That's the not any(InitializeParameterInstruction init).getParameter() = p part that's in the IPA branch for the new dataflow nodes. At least, that's what I intended when I wrote it 😂

Ah, my brain filtered that out completely because of the horrendous formatting of the diff on my small screen 🤦