[clang] [clang] Fix conflicting declaration error with using_if_exists (PR #167646)

[Mariya Podchishchaeva via cfe-commits]

Fznamznon wrote:

> @Fznamznon @Sirraide Can one of you review this when you get a chance given you are Sema maintainers? Thanks!

I'm not familiar with the attribute, so maybe @erichkeane and @AaronBallman can weigh in here but I'll try to take a look as well.
Sorry for the delay, the review notification slipped into post-vacation pile of emails. 

https://github.com/llvm/llvm-project/pull/167646