[clang] [Clang] Fix incorrect mutability of decltype((x)) in lambda return types (PR #180690)

[Samrudh Nelli via cfe-commits]

SamrudhNelli wrote:

Hi @cor3ntin @zyn0217, thank you for your help in solving the issue!! 
Since the PR is now approved, should I rebase and squash the branch? 
If not, could you please merge it when you have a moment, as I don’t have commit access to the repository? Thanks.

https://github.com/llvm/llvm-project/pull/180690