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

[Mehdi Amini via cfe-commits]

joker-eph wrote:

Seems like a bot is broken: https://lab.llvm.org/buildbot/#/builders/271/builds/7701 ; can you check?

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