Auto merge of #97652 - RalfJung:cenum_impl_drop_cast, r=nagisa

make cenum_impl_drop_cast deny-by-default

Also make it show up as future breakage diagnostic.

In rust-lang/rust#96862 we are proposing to change behavior of those drops *again*, so this looks like a good opportunity to increase our pressure on getting them out of the ecosystem. Looking at the [tracking issue](rust-lang/rust#73333), so far nobody spoke up in favor of this (accidental) feature.

Cc rust-lang/rust#73333 `@oli-obk`

Author: bors