Auto merge of #128792 - compiler-errors:foreign-sig, r=spastorino

Use `FnSig` instead of raw `FnDecl` for `ForeignItemKind::Fn`, fix ICE for `Fn` trait error on safe foreign fn

Let's use `hir::FnSig` instead of `hir::FnDecl + hir::Safety` for `ForeignItemKind::Fn`. This consolidates some handling code between normal fns and foreign fns.

Separetly, fix an ICE where we weren't handling `Fn` trait errors for safe foreign fns.

If perf is bad for the first commit, I can rework the ICE fix to not rely on it. But if perf is good, I prefer we fix and clean up things all at once 👍

r? spastorino

Fixes #128764

compiler/rustc_trait_selection/src/error_reporting/traits/fulfillment_errors.rs

@@ -2729,6 +2729,10 @@ impl<'a, 'tcx> TypeErrCtxt<'a, 'tcx> {
             | Node::ImplItem(&hir::ImplItem { kind: hir::ImplItemKind::Fn(ref sig, _), .. })
             | Node::TraitItem(&hir::TraitItem {
                 kind: hir::TraitItemKind::Fn(ref sig, _), ..
+            })
+            | Node::ForeignItem(&hir::ForeignItem {
+                kind: hir::ForeignItemKind::Fn(ref sig, _, _),
+                ..
             }) => (
                 sig.span,
                 None,