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

[Younan Zhang via cfe-commits]

zyn0217 wrote:

@ilya-biryukov I'm a bit torn about how we would proceed: if Corentin insists on the current (which my patch continues the effort) implementation, do you think it feasible for me to merge part of your codes (specifically, the way we propagate up unexpanded flags for lambda bodies, some tests apart from those for lambda attributes) into #86265? Of course I'll add you as a co-author of that patch, if you're happy.

(The attribute part could be split up as an individual PR afterward, if I understand our discussion correctly?)

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