[clang] [clang] Implement provisional wording for CWG2398 regarding packs (PR #90820)

[Mehdi Amini via cfe-commits]

joker-eph wrote:

You're right, it's visible on the link I posted, the build was already broken! Somehow I fat-fingered and didn't hit the first red build but the third one!

https://github.com/llvm/llvm-project/pull/90820