Fix handling of PM_CONSTANT_PATH_NODE node in keyword arguments with ARGS_SPLAT
This was handled correctly in parse.y (NODE_COLON2), but not in
prism. This wasn't caught earlier, because I only added tests for
the optimized case and not the unoptimized case. Add tests for
the unoptimized case.
In code terms:
```ruby
m(*a, kw: lvar::X) # Does not require allocation for *a
m(*a, kw: method()::X) # Requires allocation for *a
```
This commit fixes the second case when prism is used.
f0f97886fc