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

[Vlad Serebrennikov via cfe-commits]

Endilll wrote:

> In the proposed resolution, i do not understand why B* bp = n2p; should be ill-formed by virtue of being declared in P2

@cor3ntin Do you understand why http://eel.is/c++draft/class.protected#1 is there? (I'm trying to pick the best point to start explanation from)

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