[Clang][Sema] Convert warning for extraneous template parameter lists to an...

[Clang][Sema] Convert warning for extraneous template parameter lists to an extension warning (#82277)

We currently accept the following explicit specialization with a warning
for the extraneous template parameter list:
```
template<typename T>
void f();

template<>
template<>
void f<int>(); // warning: extraneous template parameter list in template specialization
```

This should really be an extension warning so we reject with
`-pedantic-errors`. This patch converts the warning to an extension
warning.