C++: Add an interface for models to block flow

[MathiasVP]

This PR adds a new abstract class FlowOutBarrierFunction for blocking flow out of a function model. One of the obvious examples is std::swap where we on main has this FP:

void test() {
  char* s1 = tainted();
  char* s2 = nullptr;
  std::swap(s1, s2);
  sink(s1); // should not be tainted
}

This PR adds an interface for specifying that such functions shouldn't have flow out of a given FunctionInput. Operationally, this amounts to blocking use-use flow once the data reaches the function's input.

In this PR I've just implemented the class on std::swap. Once the change is in we can provide extents for a bunch of other classes that we currently model.

[rdmarsh2]

Code and tests LGTM. There's a fair bit of noise on the DCA run, it might be worth rechecking some of the projects with bigger slowdowns.

[MathiasVP]

Code and tests LGTM. There's a fair bit of noise on the DCA run, it might be worth rechecking some of the projects with bigger slowdowns.

Thanks! I'm not sure what slowdowns you're referring to? The largest slowdown is abseil which has a 5% slowdown (49 seconds out of ~1000 seconds). I don't think this is worth digging into.

Stage timing is also showing a less-than-5% slowdown which I think we can attribute to noise