Rollup merge of #120107 - shepmaster:dead-code-repr-transparent, r=Nilstrieb

dead_code treats #[repr(transparent)] the same as #[repr(C)]

In #92972 we enabled linting on unused fields in tuple structs. In #118297 that lint was enabled by default. That exposed issues like #119659, where the fields of a struct marked `#[repr(transparent)]` were reported by the `dead_code` lint. The language team [decided](rust-lang/rust#119659 (comment)) that the lint should treat `repr(transparent)` the same as `#[repr(C)]`.

Fixes #119659

Author: matthiaskrgr