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

[Ilya Biryukov via cfe-commits]

https://github.com/ilya-biryukov commented:

Sorry for not looking closer yet, but could we get a test case?
It would make it much easier to review this change.

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