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

[Vlad Serebrennikov via cfe-commits]

Endilll wrote:

> it's clearer that it's not conforming (at the very least we should say something like "clang extension"

As someone who was a user of Clang not so long ago, I have to say that the only thing "ISO C++ requires" conveyed to me was "you're doing something wrong". For me it never had a clear interpretation "you're asking for an extension, but we're not giving it to you". On the other hand, I believe the word "extension" is clearly understood to describe something outside of ISO C++, somewhat by definition. At least that would match my understanding 3-4 years ago.

CC @AaronBallman I remember we discussed it offline a while ago.

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