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repr(transparent): do not consider repr(C) types to be 1-ZST Context: https://github.com/rust-lang/unsafe-code-guidelines/issues/552 This experiments with a [suggestion](https://github.com/rust-lang/rfcs/pull/3845#discussion_r2388463698) by ```@RustyYato``` to stop considering repr(C) types as 1-ZST for the purpose of repr(transparent). If we go with https://github.com/rust-lang/rfcs/pull/3845 (or another approach for fixing repr(C)), they will anyway not be ZST on all targets any more, so this removes a portability hazard. Furthermore, zero-sized repr(C) structs [may have to be treated](https://github.com/rust-lang/unsafe-code-guidelines/issues/552#issuecomment-3250657813) as non-ZST for the win64 ABI (at least that's what gcc/clang do), so allowing them to be ignored in repr(transparent) types is not entirely coherent. Turns out we already have an FCW for repr(transparent), namely https://github.com/rust-lang/rust/issues/78586. This extends that lint to also check for repr(C).
For high-level intro to how type checking works in rustc, see the type checking chapter of the rustc dev guide.