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

[Matheus Izvekov via cfe-commits]

mizvekov wrote:

Yep, I confirm the behavior happens if I add `-triple x86_64-windows-msvc` to RUN line.

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