[clang] [C++20] [Modules] Don't check redeclaration for TagUseKind::Referencekind declaration (PR #194546)

[Chuanqi Xu via cfe-commits]

ChuanqiXu9 wrote:

@AaronBallman @cor3ntin @mizvekov do you think this is purely a compiler bug? Or a language spec bug?

For `struct kevent evt;` clang will create a new declaration for `struct kevent`. Then the existing logic reject the example according to the spec literally. 

So either we have a bug in the wording or in the design, or clang shouldn't create a declaration for  `struct kevent evt;` in this case.

https://github.com/llvm/llvm-project/pull/194546