[clang] [Clang] Prevent assignment to captured structured bindings inside immutable lambda (PR #120849)

[via cfe-commits]

TilakChad wrote:

Thanks @cor3ntin for the review and the approval. 

I don't have commit access  as it's my very first PR. So, could you please merge this PR for me? 

https://github.com/llvm/llvm-project/pull/120849