Fix is_foreign_item for StableMIR instance

Change the implementation of `Instance::is_foreign_item` to directly
query the compiler for the instance `def_id` instead of incorrectly
relying on the conversion to `CrateItem`.

Background:

- In pull https://github.com/rust-lang/rust/pull/118524, I fixed the
  conversion from Instance to CrateItem to avoid the conversion if the
  instance didn't have a body available. This broke the `is_foreign_item`.

compiler/stable_mir/src/lib.rs

@@ -120,7 +120,7 @@ impl CrateItem {
     }
 
     pub fn is_foreign_item(&self) -> bool {
-        with(|cx| cx.is_foreign_item(*self))
+        with(|cx| cx.is_foreign_item(self.0))
     }
 
     pub fn dump<W: io::Write>(&self, w: &mut W) -> io::Result<()> {