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

[Yanzuo Liu via cfe-commits]

zwuis wrote:

I'm not sure if I should modify CWG400 implementation status to "Clang 21". CC @Endilll 

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