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

[Ilya Biryukov via cfe-commits]

ilya-biryukov wrote:

> At first glance this seems like a duplicate of #86265

That seems right, I was not aware of the other PR, thanks for pointing this out.
I still feel that avoiding the use of the flag from `LambdaScopeInfo` is an overall improvement that makes the code simpler, but the other patch seems a bit more complicated and may be addressing more issues. I'll try to run the tests added there on top of my fix.

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