Improve assertion messages in Module finalization.

When an object being finalized references an object declared as needing
a definition, the definition needs to be available. Add asserts to catch
this specific case.

lib/module/src/module.rs

@@ -263,7 +263,13 @@ where
         name: &ir::ExternalName,
     ) -> (Option<&B::CompiledFunction>, &str, &ir::Signature) {
         let info = self.contents.get_function_info(name);
+        debug_assert!(
+            !info.decl.linkage.is_definable() || info.compiled.is_some(),
+            "Finalization requires a definition for function {}.",
+            name,
+        );
         debug_assert_eq!(info.decl.linkage.is_definable(), info.compiled.is_some());
+
         (
             info.compiled.as_ref(),
             &info.decl.name,
@@ -278,7 +284,13 @@ where
         name: &ir::ExternalName,
     ) -> (Option<&B::CompiledData>, &str, bool) {
         let info = self.contents.get_data_info(name);
+        debug_assert!(
+            !info.decl.linkage.is_definable() || info.compiled.is_some(),
+            "Finalization requires a definition for data object {}.",
+            name,
+        );
         debug_assert_eq!(info.decl.linkage.is_definable(), info.compiled.is_some());
+
         (info.compiled.as_ref(), &info.decl.name, info.decl.writable)
     }