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

[Ilya Biryukov via cfe-commits]

https://github.com/ilya-biryukov approved this pull request.

LGTM from my side if all other tests pass.
Given that it's a small change and bugfix, I think it should be okay to land this without waiting for other reviewers. We could always follow up with more fixes if they have additional comments.

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