Clang C compat: Empty aggregate initializers

(Created in support of PR #235)

We noticed a regression where empty initializers for aggregates are no longer accepted when emulating clang versions before clang 17. For example, the following fails with EDG 7.0:

```
  struct A {
    int i;
  };

  struct A a = {};
```

This seems to be due to the changes made as part of EDGcpfe/29042.