Fix bug in unifying records

This was leading us to have an infinitely-recursive type, which eventually causes layout to stack-overflow

Fixes #4739

crates/compiler/test_mono/src/tests.rs

@@ -1530,6 +1530,34 @@ fn encode_derived_record() {
     )
 }
 
+#[mono_test(no_check)]
+fn choose_correct_recursion_var_under_record() {
+    indoc!(
+        r#"
+        Parser : [
+            Specialize Parser,
+            Record (List {parser: Parser}),
+        ]
+
+        printCombinatorParser : Parser -> Str
+        printCombinatorParser = \parser ->
+            when parser is
+                Specialize p ->
+                    printed = printCombinatorParser p
+                    if Bool.false then printed else "foo"
+                Record fields ->
+                    fields
+                        |> List.map \f ->
+                            printed = printCombinatorParser f.parser
+                            if Bool.false then printed else "foo"
+                        |> List.first
+                        |> Result.withDefault ("foo")
+
+        printCombinatorParser (Record [])
+        "#
+    )
+}
+
 #[mono_test]
 fn tail_call_elimination() {
     indoc!(