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

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cor3ntin 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.

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