Support for DR2936: Local classes of templated functions should be part of the current instantiation

Test code ([CE](https://godbolt.org/z/7r76s9h55)):

```cpp
template void f() {
// - EDG: OK
// - GCC/Clang/MSVC: error
[] {}.operator()<0>();
}
```

According to [[temp.dep.type]/10](https://eel.is/c++draft/temp.dep.type#10), the closure type above is not dependent. Even if it happens to be dependent, [[temp.dep.type]/1.5](https://eel.is/c++draft/temp.dep.type#1.5) (added by [CWG2936](https://cplusplus.github.io/CWG/issues/2936.html)) specifies that a local class in a templated function is part of the current instantiation (though I'm not sure whether this applies to closure types):

> - in the definition of a templated function, the name of a local class ([class.local]).

So, I don't think the `template` disambiguator should be required here, as it only adds syntactic noise.