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

[Ilya Biryukov via cfe-commits]

ilya-biryukov wrote:

> r3 is in 2 weeks and we have to be more conservatives in our backports, so it really depends on how long it took you and @ilya-biryukov to address these issues and the complexity of the patches.
> 
> In particular, I would be surprised if the attribute case impacts many people.

To be clear, I don't have a real-world use-case for attributes. The only reason I have added it in my patch is because there was a test in Clang that started failing with my approach (because it started looking at thing structurally even on the parsing path, as discussed earlier).
I suspect we could live with that for a long time, although fixing that eventually is a good idea. (And is still in my TODO list, just slipped a little)

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