Rollup merge of #129199 - RalfJung:writes_through_immutable_pointer, r=compiler-errors

make writes_through_immutable_pointer a hard error

This turns the lint added in rust-lang/rust#118324 into a hard error. This has been reported in cargo's future-compat reports since Rust 1.76 (released in February). Given that const_mut_refs is still unstable, it should be impossible to even hit this error on stable: we did accidentally stabilize some functions that can cause this error, but that got reverted in rust-lang/rust#117905. Still, let's do a crater run just to be sure.

Given that this should only affect unstable code, I don't think it needs an FCP, but let's Cc ``@rust-lang/lang`` anyway -- any objection to making this unambiguous UB into a hard error during const-eval? This can be viewed as part of rust-lang/rust#129195 which is already nominated for discussion.

Author: matthiaskrgr