[clang] [Clang] Fix handling of pack indexing types in constraints of redeclaration (PR #139057)

[Erich Keane via cfe-commits]

https://github.com/erichkeane approved this pull request.

On its face, and thinking about it for ~10 minutes, this seems completely reasonable.  I think @zyn0217 has some reasonable comments, so make him happy, and I am.

https://github.com/llvm/llvm-project/pull/139057