Rollup merge of #147185 - RalfJung:repr-c-not-zst, r=petrochenkov

repr(transparent): do not consider repr(C) types to be 1-ZST

Context: rust-lang/unsafe-code-guidelines#552

This experiments with a [suggestion](rust-lang/rfcs#3845 (comment)) by ```@RustyYato``` to stop considering repr(C) types as 1-ZST for the purpose of repr(transparent). If we go with rust-lang/rfcs#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](rust-lang/unsafe-code-guidelines#552 (comment)) 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 rust-lang/rust#78586. This extends that lint to also check for repr(C).

Author: Zalathar