Fix unused_unsafe with compiler-generated unsafe

compiler/rustc_mir/src/transform/check_unsafety.rs

@@ -321,6 +321,7 @@ impl<'a, 'tcx> UnsafetyChecker<'a, 'tcx> {
                 }
                 false
             }
+            Safety::BuiltinUnsafe => true,
             Safety::ExplicitUnsafe(hir_id) => {
                 // mark unsafe block as used if there are any unsafe operations inside
                 if !violations.is_empty() {