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

[Matheus Izvekov via cfe-commits]

================

----------------
mizvekov wrote:

I had some discussion about that with @Endilll on the previous MR regarding this core issue: https://github.com/llvm/llvm-project/pull/89807

Since there is no posting at all in core about any possible solutions, I wanted to get feedback from them before claiming to be implementing a solution in the public documentation.

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

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