[clang] [Sema] Fix computations of "unexpanded packs" in substituted lambdas (PR #99882)

[Ilya Biryukov via cfe-commits]

ilya-biryukov wrote:

> This would be excellent, but I think we would still need that just because building a lambda is an horrible state machine.

Yeah, unfortunately there probably isn't a way to get rid of the state machine.
I can only see a path towards getting rid of that single flag. But that's a problem for another day...

https://github.com/llvm/llvm-project/pull/99882