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

[via cfe-commits]

NeKon69 wrote:

> So the advantage is that this gives us the freedom to change the implementation details more freely to independent attributes if we see fit, and it gives a more consistent user experience, as in it preserves the original behaviour of not allowing duplicates of the same spelling.

I see. Do you want me to do it in this patch?

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