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

[Hans Wennborg via cfe-commits]

zmodem wrote:

> So if you want to go ahead with your revert, let me know.

Let's see what the others think during the day as well. I don't want to force things in any direction.

> 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?

It's a pretty mixed group in the discussion so far, so let's keep it upstream :)

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