Rollup merge of #126881 - WaffleLapkin:unsafe-code-affected-by-fallback-hard-in-2024, r=compiler-errors

Make `NEVER_TYPE_FALLBACK_FLOWING_INTO_UNSAFE` a deny-by-default lint in edition 2024

I don't actually really care about this, but ``@traviscross`` asked me to do this, because lang team briefly discussed this before.

(TC here:)

Specifically, our original FCPed plan included this step:

- Add a lint against fallback affecting a generic that is passed to an `unsafe` function.
   - Perhaps make this lint `deny-by-default` or a hard error in Rust 2024.

That is, we had left as an open question strengthening this in Rust 2024, and had marked it as an open question on the tracking issue.  We're nominating here to address the open question.  (Closing the remaining open question helps us to fully mark this off for Rust 2024.)

r? ``@compiler-errors``

Tracking:

- rust-lang/rust#123748

Author: jieyouxu