[clang] [Clang][Sema] Retain the expanding index for unevaluated type constraints (PR #109518)

[Erich Keane via cfe-commits]

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

I like the solution here, I think this is a good way of making our constraint instantiation more stable.  Thanks!

https://github.com/llvm/llvm-project/pull/109518