fix ICE when using #[no_mangle] on a generic item

Author: folkertdev

tests/pass/issues/issue-154385-no-mangle-generic.rs

@@ -1,9 +1,22 @@
 fn main() {
-    foo(1234);
+    generic_type(123);
+    generic_const::<456>();
+    generic_lifetime(&789);
 }
 
 #[allow(no_mangle_generic_items)]
 #[unsafe(no_mangle)]
-fn foo<T: std::fmt::Debug>(value: T) {
+fn generic_type<T: std::fmt::Debug>(value: T) {
     println!("{value:?}");
 }
+
+#[expect(no_mangle_generic_items)]
+#[unsafe(no_mangle)]
+fn generic_const<const N: usize>() {
+    println!("{N}");
+}
+
+#[unsafe(no_mangle)]
+fn generic_lifetime<'a>(x: &'a i32) {
+    println!("{x}");
+}

tests/pass/issues/issue-154385-no-mangle-generic.stdout

@@ -1 +1,3 @@
-1234
+123
+456
+789