[clang] [SemaCXX] Implement CWG2351 `void{}` (PR #78060)

[Vlad Serebrennikov via cfe-commits]

Endilll wrote:

> Looking at other places, it looks like init-list stuff is guarded behind getLangOpts().CPlusPlus11, so I'll add that check.

Corentin told me offline that check for list initialization that you do might be sufficient, as it can't pass in 98.

> It looks like this DR is CD5 (after C++17, applies to C++17), but void{} in C++11/14 without a warning seems fine.

Yes, 11 and 14 are totally fine. We may want a pedantic warning, though.

> As for "regular tests", do you mean in [/clang/test/CXX](https://github.com/llvm/llvm-project/tree/main/clang/test/CXX)? Should I also add tests there?

No, this is C++ conformance test suite. I meant `clang/test/SemaCXX`.

https://github.com/llvm/llvm-project/pull/78060