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

[Matheus Izvekov via cfe-commits]

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mizvekov wrote:

Note we are not implementing the solution Jason posted on the core mailing list, neither on the previous patch, as we have a better solution than current GCC on this, nor on this MR, as GCC implements no such workaround and still fails this test.

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