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

[via cfe-commits]

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

----------------
h-vetinari wrote:

AFAIU, for this to be picked up by [make_cxx_dr_status](https://github.com/llvm/llvm-project/blob/main/clang/www/make_cxx_dr_status) correctly, this file should have a comment near the top like:
```
// cwg2398: 19 drafting 2016-12-03
```
and for consistency should be mentioned in `dr23xx.cpp` like so:
https://github.com/llvm/llvm-project/blob/b86e0992bfa6c58be077d82d824016f590ac5d90/clang/test/CXX/drs/dr23xx.cpp#L276

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