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

[Vlad Serebrennikov via cfe-commits]

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Endilll wrote:

> Also, this needs to test a non-conforming mode until we finally remove the corresponding flag, and that is against the rules for the DR suite.

You should be able to split conforming part of the test into DR test, though.

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