Rollup merge of #161745 - RalfJung:repr-exhaustive, r=mati865

make trivial ABI check resilient against new repr

There's currently no way to exhaustively match on "what basic `repr` is this", which is kind of fragile as it means a new repr will get whatever the `else` branch happens to be everywhere. Changing this would be a major refactor, but let's at least make it possible to `assert!` that we covered all the cases by adding an explicit way to check for `repr(Rust)`.