Auto merge of #84267 - dtolnay:ptrunit, r=nagisa

Make *const (), *mut () okay for FFI

Pointer-to-() is used occasionally in the standard library to mean "pointer to none-of-your-business". Examples:

- `RawWakerVTable::new` https://doc.rust-lang.org/1.51.0/std/task/struct.RawWakerVTable.html#method.new
- `<*const T>::to_raw_parts` https://doc.rust-lang.org/nightly/std/primitive.pointer.html#method.to_raw_parts

I believe it's useful for the same purpose in FFI signatures, even while `()` itself is not FFI safe. The following should be allowed:

```rust
extern "C" {
    fn demo(pc: *const (), pm: *mut ());
}
```

Prior to this PR, those pointers were not considered okay for an extern signature.

```console
warning: `extern` block uses type `()`, which is not FFI-safe
 --> src/main.rs:2:17
  |
2 |     fn demo(pc: *const (), pm: *mut ());
  |                 ^^^^^^^^^ not FFI-safe
  |
  = note: `#[warn(improper_ctypes)]` on by default
  = help: consider using a struct instead
  = note: tuples have unspecified layout

warning: `extern` block uses type `()`, which is not FFI-safe
 --> src/main.rs:2:32
  |
2 |     fn demo(pc: *const (), pm: *mut ());
  |                                ^^^^^^^ not FFI-safe
  |
  = help: consider using a struct instead
  = note: tuples have unspecified layout
```

compiler/rustc_lint/src/types.rs

@@ -1056,6 +1056,15 @@ impl<'a, 'tcx> ImproperCTypesVisitor<'a, 'tcx> {
                 FfiSafe
             }
 
+            ty::RawPtr(ty::TypeAndMut { ty, .. })
+                if match ty.kind() {
+                    ty::Tuple(tuple) => tuple.is_empty(),
+                    _ => false,
+                } =>
+            {
+                FfiSafe
+            }
+
             ty::RawPtr(ty::TypeAndMut { ty, .. }) | ty::Ref(_, ty, _) => {
                 self.check_type_for_ffi(cache, ty)
             }