[rs4gc] Strip nofree and nosync attributes when lowering from abstract model

The safepoints being inserted exists to free memory, or coordinate with another thread to do so.  Thus, we must strip any inferred attributes and reinfer them after the lowering.

I'm not aware of any active miscompiles caused by this, but since I'm working on strengthening inference of both and leveraging them in the optimization decisions, I figured a bit of future proofing was warranted.