Copy the abstract-value closure conversion pass

Copy the existing abstract-value closure conversion pass, so that it can
remain in the pipeline until the new multi-entry pipeline is set up.

This commit will be reverted once the multi-entry pipeline is set up.
The closure converter now produces multi-entry SSA code

Going from SXML -> SSA to SXML -> ME_SSA is pretty straightforward.
Since SXML functions don't have multiple entries and this pass doesn't
add entry points to functions, when SSA functions are created they now
create a single entry point and a name for that entry point.