[clang] [Clang] Introduce FunctionParmPackDecl for expanded lambda captures (PR #107995)

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cor3ntin wrote:

> @cor3ntin I thought introducing a new Decl node only to ensure the invariance of the protocol of TransformDecl() seemed a bit waste, but I don't know if there's a better approach, so I left it as a draft.

This looks like a reasonable approach to me

https://github.com/llvm/llvm-project/pull/107995