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

[Corentin Jabot via cfe-commits]

cor3ntin wrote:

Given the DR was voted in 2004 it's clearly meant to apply to C++98.
GCC rejects the code, so it seems reasonable to do the same, and it's a nice code simplification.
https://compiler-explorer.com/z/heTzG9hxh

I don't think we need to change the dr status page 

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