Rollup merge of #120805 - RalfJung:const-pat-partial-eq, r=oli-obk

make non-PartialEq-typed consts as patterns a hard error

This lint was introduced in rust-lang/rust#115893, for Rust 1.74, so we just had the third stable release where this is shown as a future-compat lint (which is shown for dependencies). Not a single comment or backreference showed up in the tracking issue, rust-lang/rust#116122. So this seems fairly safe to turn into a hard error.

Of course we should do a crater run first.

This is part of rust-lang/rust#120362.
Closes rust-lang/rust#116122.

Author: matthiaskrgr