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

[Younan Zhang via cfe-commits]

zyn0217 wrote:

@erichkeane : I've updated the patch to limit the scope to lambda-only, which is now more appropriate to me. PTAL, thanks!

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