Clang accepts a default template argument on a variable-template partial specialization

Clang 22 accepts the following C++20 program, while Clang GCC 15.2.0 rejects it:

```c++
template
int var_template = 0;

template
int var_template;

int *p = &var_template;
```

Clang: clang++-22 22.0.0
-std=c++20: accepts
-std=c++20 -pedantic-errors: accepts

GCC: gcc15 15.2.0
-std=c++20: rejects
-std=c++20 -pedantic-errors: rejects

GCC diagnostic:
error: default template arguments may not be used in partial specializations