[clang] [Clang][Sema] Diagnose variable template explicit specializations with storage-class-specifiers (PR #93873)

[Matheus Izvekov via cfe-commits]

https://github.com/mizvekov commented:

Interestingly, only GCC errors on a case like this:

```
namespace {
  template<class T> int A = 0;
}
template<> int A<void> = 0;
```

https://godbolt.org/z/TTjssKxz5

https://github.com/llvm/llvm-project/pull/93873