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

[Shafik Yaghmour via cfe-commits]

https://github.com/shafik commented:

@Endilll has been handling a lot of conformance testing, so please wait for more feedback from him before landing.

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