[Clang] C23 Fixed-type enum specifier rejected in _Generic association list

```C
int main() {
// clang error: unnamed enumeration must be a definition
(void)_Generic(0, enum:long{A}:1, default: 0);

// clang error: expected expression
(void)_Generic(0, enum E:long{B}:1, default: 0);
}
```
GCC accepts both : https://c.godbolt.org/z/GdhcWx5hM

Start by compiling the two C23 _Generic reproductions from the issue and compare Clang's behavior with GCC. Trace the parser or semantic handling of fixed-type enum specifiers in association lists; done means both forms are accepted and covered by regression tests.

Written by the indexing model from the issue text.