Pouring concrete for an under porch crawl space
The generic term for non-structural pours like that is rat slab, regardless of whether it's for rodent control or not. There doesn't seem to be any logical call for a liner, but local codes and ordinances aren't always logical. There are a few caveats besides that to keep in mind.


You'll want a slab at least two inches thick, three would be better. Don't be the guy that makes due with one inch and has it fail three weeks later.
I'd pitch the slab away from the house; no need intentionally create drainage nightmares.
The point where you want to have the concrete trucked in approaches a whole lot quicker in a crawlspace because lugging buckets on hands and knees sucks. A lot.