The slot is now always known (at the root of the file it is simply
stack slot 0 once the scope drops back down to 0), so it does not need
to be wrapped in an `Option` and accessed in cumbersome ways anymore.
Change-Id: I46bf67a4cf5cb96e4874dffd0e3fb07c551d44f0
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6420
Reviewed-by: sterni <sternenseemann@systemli.org>
Tested-by: BuildkiteCI