[clang] [Clang][Sema] Apply CWG400 'Using-declarations and the "struct hack"' to C++98 mode (PR #143492)

[Vlad Serebrennikov via cfe-commits]

Endilll wrote:

Tests in this PR needs to be evaluated against the existing wording, and not just against 20-years-old wording. I'm still not finished with that.

https://github.com/llvm/llvm-project/pull/143492