Fix #8487: Debugger confuses variables

To display the free variables for a single breakpoint, GHCi pulls out the information from the fields modBreaks_breakInfo and modBreaks_vars of the ModBreaks data structure. For a specific breakpoint this gives 2 lists of type Id and OccName. They are used to create the Id's for the free variables and must be kept in sync. If we remove an element from the Id list, then we also must remove the corresponding element from the OccName list.