[clang] [Clang][Sema] Avoid duplicate diagnostics for incomplete types in nested name specifier (C++20+) (PR #147036)

[Shafik Yaghmour via cfe-commits]

shafik wrote:

> Why doesn't C++20 below suffer from the issue?
> 
> I think it's a waste to add a Sema scope object just for diagnostic issues. It's more like that there's an underlying issue that would be otherwise hidden by the patch. Can you explore?

Thank you for this PR but I agree this does not sound like the right approach and it looks like we have another candidate path in the pipeline already.

https://github.com/llvm/llvm-project/pull/147036