lint(unsafe_code): exclude unsafe declarations from lint coverage

[wyfo]

Unsafe declarations of traits, functions, methods, or extern blocks cannot cause undefined behavior by themselves — only unsafe blocks or implementations of unsafe traits can.

The unsafe_code lint previously applied to these declarations, preventing, for example, declarations of extern "C" blocks. See #108926.

This change removes all such unsafe declarations from unsafe_code coverage.

However, to maintain soundness, unsafe functions with bodies are only allowed when unsafe_op_in_unsafe_fn is set to forbid. This ensures unsafe operations inside such functions require an unsafe block. Declarations of unsafe functions without bodies (e.g., in traits) are always allowed.

Closes #108926

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  • a4b70f2

[RalfJung]

Unsafe declarations of traits, functions, methods, or extern blocks cannot cause undefined behavior by themselves

At least for extern blocks, that is not correct. The mere presence of an incorrect extern block can cause UB. See #46188.

[wyfo]

At least for extern blocks, that is not correct. The mere presence of an incorrect extern block can cause UB. See #46188.

Indeed, my bad, I've rolled back this part.