optbuilder: star expansion in UDFs produces duplicate column names

When a routine is created, we expand the star expressions (all-columns selector) into the referenced column names. However, the following example shows a case where we generate duplicate names, which leads to an error upon trying to resolve the expanded query:
```
CREATE FUNCTION f() RETURNS RECORD LANGUAGE SQL AS $$ SELECT * FROM UNNEST('{"a","b","c"}'::TEXT[], '{"d", "e","f"}'::TEXT[]) $$;
ERROR: column reference "unnest" is ambiguous (candidates: .unnest)
SQLSTATE: 42702
```

Jira issue: CRDB-32684