[clang] [Sema] Preserve ContainsUnexpandedParameterPack in TransformLambdaExpr (PR #86265)

[Ilya Biryukov via cfe-commits]

ilya-biryukov wrote:

Just FYI, I wasn't aware of this PR and have ended up with an alternative approach to tracking the unexpanded packs in #99882. (Thanks @cor3ntin for pointing this out)

I'll let the reviewers decide if we should go with this change or mine, also happy to have some combination of the two (e.g. rebase mine on top of this one and still implement some of the refactorings there).

https://github.com/llvm/llvm-project/pull/86265