MIR MutVisitor::visit_local cannot mutate all Locals #71008
Description
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.