Note, my instinct is that `Float(dyn_size)` should be invalid. However,
I think the constant evaluation doesn't result in the call being
evaluated in eval.cpp when the size is non-constant. I think I could get
an error for symbolic phase calls, but that seems a little less
interesting already. Long-term maybe we want a way to mark functions as
_must_ be evaluated during constant phase?
Also, I think there may be a bug with literal value parse node
locations, I should be able to point at the position of `arg_ids[0]` but
it's missing a line number so I point at `loc` instead.