Auto merge of #118482 - RalfJung:interpret-local-type, r=WaffleLapkin

explain a good reason for why LocalValue does not store the type of the local

As found out by `@lcnr` in rust-lang/rust#112307, storing the type here can lead to subtle bugs when it gets out of sync with the MIR body. That's not the reason why the interpreter does it this way I think, but good thing we dodged that bullet. :)