MIR `MutVisitor::visit_local` cannot mutate all `Local`s

Specifically, when a `ProjectionElem::Index` is encountered, the contained `Local` is not passed to `visit_local`, requiring the `MutVisitor` to *also* implement `process_projection_elem`.

This is a footgun that may cause unsound MIR transforms, which is what happened to me while working on #71003. The `CopyProp` pass also has to explicitly implement `process_projection_elem` to be sound.