[clang] [clang][Sema] Deprecate `global`/`unknown` in `lifetime_capture_by` (PR #196635)

[Corentin Jabot via cfe-commits]

cor3ntin wrote:

@zmodem I'm very tempted to approve your revert PR. I think we could have done a better job mitigating the disruption, especially given we are about to branch clang 23. So if you want to go ahead with your revert, let me know.

However, given that most of the stakeholders seem to work for Google, it might be useful to see if you can come up with some sort of agreement internally?

https://github.com/llvm/llvm-project/pull/196635