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

[Matheus Izvekov via cfe-commits]

mizvekov wrote:

Weirdly enough the test passes on my machine, latest MacOS.

Maybe the test is not constrained on target, and this is causing differences between machines?

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