Rollup merge of #56594 - sdroege:c_void-is-not-never, r=TimNN

Remove confusing comment about ideally using `!` for `c_void`

Using `!` for `c_void` would have the problem that pointers and
potentially references to an uninhabited type would be created, and at
least for references this is UB.

In addition document that newtype wrappers around `c_void` can be used
safely in place of `extern type` until the latter is stabilized.

----

I'm not 100% sure about the usage for opaque types as the [nomicon](https://doc.rust-lang.org/nomicon/ffi.html#representing-opaque-structs) still recommends using `#[repr(C)] pub struct Foo { _private: [u8; 0] }` but it seems like these two should be equivalent in the end? Also the `#[repr(C)]` (in both cases) should be unneeded because such types never being passed by value, never being dereferenced but only passed around as pointer or reference, so the representation of (*values* of) the type itself should not matter at all?

Also in context of `c_void` and `!` the second unresolved question in the [`extern type`](https://github.com/rust-lang/rust/issues/43467) stabilization ticket seems relevant

> In [std's](https://github.com/rust-lang/rust/blob/164619a8cfe6d376d25bd3a6a9a5f2856c8de64d/src/libstd/os/raw.rs#L59-L64) source, it is mentioned that LLVM expects i8* for C's void*.
> We'd need to continue to hack this for the two c_voids in std and libc.
> But perhaps this should be done across-the-board for all extern types?
> Somebody should check what Clang does.

Please correct me if my understanding is wrong and everything's actually fine as is.

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