[Clang] Clang incorrectly accepts `volatile volatile;` as a statement

Clang accepts the following C program without any diagnostics:

```
int a() {
volatile volatile;
}
int main() {}
```

GCC correctly rejects this one. Please refer to the link: https://godbolt.org/z/eq6e8o67o

Start by compiling the C reproducer from the issue with Clang and GCC, and compare the diagnostics with the linked Godbolt example. No source file or test is named; the work is done when Clang rejects `volatile volatile;` as a statement with an appropriate diagnostic.

Written by the indexing model from the issue text.