[clang] [clang] Do not substitute parameter pack while retaining the pack expansion (PR #108197)

[Utkarsh Saxena via cfe-commits]

usx95 wrote:

Thanks for the review. Landing now. For other reviewers, feel free to drop more comments, and I would be happy to address in a followup.

https://github.com/llvm/llvm-project/pull/108197