This early validation uses interpolation of a placeholder value to achieve
some "best effort" validation of the validity of the count attribute.
Since HCL2-specified resources can't be interpolated using the main
interpolator, here we branch and use the HCL2 API to do a
largely-equivalent (though slightly less accurate) check.
In the long run we don't really need this extra check at all, since the
validation walk does a more accurate version of the same thing. However,
we're preserving this for now in the interests of minimizing the amount
of change for the main codepath during our experiment.
Martin Atkins