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

[Vlad Serebrennikov via cfe-commits]

Endilll wrote:

> Despite CWG2398 not being voted in yet, the status in
> 
> https://github.com/llvm/llvm-project/blob/e3f42b02a4129947ca2dd820bfb63ffed83027b7/clang/www/cxx_dr_status.html#L14195-L14200
> 
> should be updated.

The only way to update it is to add a test to `clang/test/CXX/drs`. Me and the author had a discussion couple of weeks ago, and the conclusion was that what he implements is not (yet) documented in the CWG2398 filing, so we don't have anything to write a DR test and claim conformance against. If something has changes since then, we should revisit that conclusion.

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