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

[Younan Zhang via cfe-commits]

https://github.com/zyn0217 commented:

Thanks for working on this.

I did a rough look, and I think the general approach looks good. Although this unfortunately happens to be a duplicate work of #86265 (sorry I didn't get around to that during the 19 development cycle due to my timeframe), I think it's fine to follow this patch because it will tidy up things like how `ContainsUnexpandedPacks` are propagated and could avoid scattered flag setting, just as the PR said.

As @cor3ntin is our lambda's code owner, I'd like to leave the approval to him.

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