Rollup merge of rust-lang#128735 - jieyouxu:pr-120176-revive, r=cjgillot

Add a special case for `CStr`/`CString` in the `improper_ctypes` lint

Revives rust-lang#120176. Just needed to bless a test and fix an argument, but seemed reasonable to me otherwise.

Instead of saying to "consider adding a `#[repr(C)]` or `#[repr(transparent)]` attribute to this struct", we now tell users to "Use `*const ffi::c_char` instead, and pass the value from `CStr::as_ptr()`" when the type involved is a `CStr` or a `CString`.

The suggestion is not made for `&mut CString` or `*mut CString`.

r? ``````@cjgillot`````` (since you were the reviewer of the original PR rust-lang#120176, but feel free to reroll)

core/src/ffi/c_str.rs

@@ -91,6 +91,7 @@ use crate::{fmt, intrinsics, ops, slice, str};
 /// [str]: prim@str "str"
 #[derive(PartialEq, Eq, Hash)]
 #[stable(feature = "core_c_str", since = "1.64.0")]
+#[rustc_diagnostic_item = "cstr_type"]
 #[rustc_has_incoherent_inherent_impls]
 #[lang = "CStr"]
 // `fn from` in `impl From<&CStr> for Box<CStr>` current implementation relies