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

[Matheus Izvekov via cfe-commits]

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mizvekov wrote:

It would not be splitting though, it would be wholesale duplication in that case.
This file is the only part of the suite still testing the old non-conformant mode, and I fail to see a test case we wouldn't be wanting to test on both.

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