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

[Vlad Serebrennikov via cfe-commits]

Endilll wrote:

> do we have test that this is fine?
> 
> ```
> namespace foo {
>     int main();
>     using a = decltype(&main);
> }```
> ```

Thank you, I added the tests.

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