Auto merge of rust-lang#12804 - B14CK313:master, r=y21

fulfill expectations in `check_unsafe_derive_deserialize`

The utility function `clippy_utils::fulfill_or_allowed` is not used because using it would require to move the check for allowed after the check iterating over all inherent impls of the type, doing possibly unnecessary work.
Instead, `is_lint_allowed` is called as before, but additionally, once certain that the lint should be emitted, `span_lint_hir_and_then` is called instead of `span_lint_and_help` to also fulfill expectations.

Note: as this is my first contribution, please feel free to nitpick or request changes. I am happy to adjust the implementation.

fixes: rust-lang#12802

changelog: fulfill expectations in [`unsafe_derive_deserialize`]

Author: bors