make repr_transparent_non_zst_fields a hard error

[RalfJung]

This was a deny-by-default FCW since #147185, which landed almost 6 months ago and shipped with Rust 1.93. (If this PR lands now it will ship with 1.96.) Already back then we found hardly any crater impact. The tracking issue has had no new relevant backrefs since that PR landed.

So, I think it is time to make this a hard error. Will nominate for t-lang once the crater run is complete.
Fixes #78586 (tracking issue)
Fixes rust-lang/unsafe-code-guidelines#552 because this means the repr(transparent) ABI compatibility rule no longer ever "ignores" repr(C) fields.

[rustbot]

r? @jdonszelmann

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[RalfJung]

@bors try

[rust-bors]

☀️ Try build successful (CI)
Build commit: 6757d70 (6757d700f93f6d16c8b39cf79e96b019bd570e7d, parent: 12f35ad39ed3e39df4d953c46d4f6cc6c82adc96)

[RalfJung]

@craterbot check

[craterbot]

👌 Experiment pr-155299 created and queued.
🤖 Automatically detected try build 6757d70
⚠️ Try build based on commit 5b074cf, but latest commit is 3bb6c15. Did you forget to make a new try build?
🔍 You can check out the queue and this experiment's details.

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