Reuse symbol when opaque type wraps a known symbol

Opaques decay immediately into their argument during codegen, so we need
to handle something that's effectively variable aliasing correctly.

This bug popped up while migrating all current private tags to opaques.

compiler/test_mono/src/tests.rs

@@ -1330,6 +1330,23 @@ fn specialize_ability_call() {
     )
 }
 
+#[mono_test]
+fn opaque_assign_to_symbol() {
+    indoc!(
+        r#"
+        app "test" provides [ out ] to "./platform"
+
+        Variable := U8
+
+        fromUtf8 : U8 -> Result Variable [ InvalidVariableUtf8 ]
+        fromUtf8 = \char ->
+            Ok ($Variable char)
+
+        out = fromUtf8 98
+        "#
+    )
+}
+
 // #[ignore]
 // #[mono_test]
 // fn static_str_closure() {