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

[Aaron Ballman via cfe-commits]

AaronBallman wrote:

What is being fixed here? We already issue a diagnostic: https://godbolt.org/z/EsjjjjYaM

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