In cases where there are positional parameters after a splat parameter,
don't attempt to match the splat parameter to a splat argument. We need
more sophisticated modelling to handle these cases, which is future
work.
This models flow in the following case:
def foo(x, y)
sink x # 1
sink y # 2
end
args = [source 1, source 2]
foo(*args)
We do this by introducing a SynthSplatParameterNode which accepts
content from the splat argument, if one is given at the callsite.
From this node we add read steps to each positional parameter.
