Expand kwcall lowering positional default check to vararg
Fixes the case from #50518, but we actually have two test cases in
the tests that also hit this (e.g. this one:
```
f40964(xs::Int...=1; k = 2) = (xs, k)
```), but just happened not to hit the bad codegen path. #50556,
once merged would have complained on those definitions as well,
without this fix.
