[clang] [Concepts] Add Decls from the outer scope of the current lambda for conversion function constraints (PR #83420)

[Erich Keane via cfe-commits]

https://github.com/erichkeane commented:

I think this looks good, but I want @cor3ntin to do the final approval, he touched it last :) 

https://github.com/llvm/llvm-project/pull/83420