[clang-tools-extra] [llvm] [clang] [clang] Add test for CWG472 (PR #67948)

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cor3ntin wrote:

After additional archeology, I found the following minutes from Portland , 2012

> Core issue 472: Casting across protected inheritance
> _ Would the example work if P2 derived privately from N2?
> _ ... Yes.. Hm, that was a good point.
> redrafting.


Given that, I'd rather we don't try to read the tea leaves.
If we think this is important enough to fix, we should ping CWG.
But there seems to be some consistency across implementations, so I think we should punt work on this issue.

@zygoloid 


https://github.com/llvm/llvm-project/pull/67948