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

[Vlad Serebrennikov via cfe-commits]

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Endilll wrote:

> Sounds like should perhaps note that we are implementing our own resolution, until there's an update to the cwg issue that can be referred to?

Our tooling is not ready for describing own own resolutions, unfortunately. Ideally if we have a resolution, it should be submitted to CWG and documented in the issue, making it easy to reference.

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