set `slot_syms` for methods of OCs constructed via `Core.OpaqueClosure` (#53650)
Previously `oc` constructed via `Core.OpaqueClosure` does not set
`oc.source.slot_syms` set, which caused segfaults either when trying to
`show` `oc.source` or if invocation of `oc(...)` threw an error. This
commit fixes that by making sure `oc.source.slot_syms` is set for `oc`
created with `jl_new_opaque_closure_from_code_info`.