[clang] [NFC][clang][Modules] Add regression test for implicit lambda capture from merged module copies (PR #222020)

[Adarsh Mishra via cfe-commits]

Adarsh-Me wrote:

@ChuanqiXu9 you mentioned on 09-23 that you were going to land this — no urgency, just checking whether anything is blocked on my side. `mergeable_state` is `blocked` (review count, not a conflict), and the earlier red on `609cb782` was `check-cross-project` infra, unrelated to this diff. Happy to rebase or re-run anything you want before it goes in.

https://github.com/llvm/llvm-project/pull/222020