[clang] [Clang] fix crash with generic lambdas in function default args (PR #222987)

[Younan Zhang via cfe-commits]

https://github.com/zyn0217 approved this pull request.

Thanks, I think the cleanup makes sense, everything seems clearer but I'd like to ask you to wait for others feedback (for 2~3 days) before going ahead.

CC @cor3ntin @mizvekov 

https://github.com/llvm/llvm-project/pull/222987