librustc: De-`@mut` `RegionVarBindings::constraints`
librustc: De-`@mut` `RegionVarBindings::skolemization_count`
librustc: De-`@mut` `RegionVarBindings::bound_count`
librustc: De-`@mut` `glbs` and `lubs` in `RegionVarBindings`
librustc: De-`@mut` the undo log in `RegionVarBindings`