Make sure crates not opting in to staged_api don't use staged_api
This also fixes the problem that with `-Zforce-unstable-if-unmarked` set, crates could not use `#[deprecated]`.
If you prefer, I can instead submit another version which just fixes this problem, but still allows the staged API attributes for all crates when `-Zforce-unstable-if-unmarked` is set. I have prepared that at <https://github.com/RalfJung/rust/tree/staged2>. As yet another alternative, @alexcrichton suggested to turn this error into a lint, but that seems to be much more work, so is it worth it?
Cc @alexcrichton #43975
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