[clang] [clang] Add test for CWG2811 "Clarify "use" of main" (PR #96168)

[via cfe-commits]

cor3ntin wrote:

I think I prefer the current wording, it's clearer that it's not conforming (at the very least we should say something like "clang extension". and maybe "referring to main is a clang extension".
LGTM otherwise

do we have test that this is fine?

```
namespace foo {
    int main();
    using a = decltype(&main);
}```

https://github.com/llvm/llvm-project/pull/96168