The loader state was stored into HscEnv. As we need to have two interpreters and one loader state per interpreter in #14335, it's natural to make the loader state a field of the Interp type.
As a side effect, many functions now only require a Interp parameter instead of HscEnv. Sadly we can't fully free GHC.Linker.Loader of HscEnv yet because the loader is initialised lazily from the HscEnv the first time it is used. This is left as future work.
HscEnv may not contain an Interp value (i.e. hsc_interp :: Maybe Interp). So a side effect of the previous side effect is that callers of the modified functions now have to provide an Interp. It is satisfying as it pushes upstream the handling of the case where HscEnv doesn't contain an Interpreter. It is better than raising a panic (less partial functions, "parse, don't validate", etc.).