ffi-polonius: Remove usage of extern types.

This will allow us to revert our dependency on extern types, which would
help our godbolt build as well as our various builders.

gcc/rust/ChangeLog:

	* checks/errors/borrowck/ffi-polonius/src/gccrs_ffi.rs: Remove extern
	type feature.
	* checks/errors/borrowck/ffi-polonius/src/lib.rs: Define FFIVector
	per the nomicon's recommendation
	https://doc.rust-lang.org/nomicon/ffi.html#representing-opaque-structs

Author: CohenArthur

gcc/rust/checks/errors/borrowck/ffi-polonius/src/gccrs_ffi.rs

@@ -30,11 +30,16 @@
 // ```
 include!("gccrs_ffi_generated.rs");
 
+use std::marker::{PhantomData, PhantomPinned};
+
 use crate::GccrsAtom;
 
-// Using opqaue types
-extern "C" {
-    pub type FFIVector;
+// We define an opaque C type per the nomicon's recommendation:
+// https://doc.rust-lang.org/nomicon/ffi.html#representing-opaque-structs
+#[repr(C)]
+pub struct FFIVector {
+    _empty: [u8; 0],
+    marker: PhantomData<(*mut u8, PhantomPinned)>,
 }
 
 impl<T1, T2> Into<(GccrsAtom, GccrsAtom)> for Pair<T1, T2>

gcc/rust/checks/errors/borrowck/ffi-polonius/src/lib.rs

@@ -16,8 +16,6 @@
 // along with GCC; see the file COPYING3.  If not see
 // <http://www.gnu.org/licenses/>.
 
-#![feature(extern_types)]
-
 mod gccrs_ffi;
 
 use gccrs_ffi::FFIVector;