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

[Utkarsh Saxena via cfe-commits]

usx95 wrote:

> (Maybe @usx95 can fix Abseil :-) I understand that it's not desirable to keep adding flags all the time, but it would be helpful for us.

​I can try to update Abseil. Should make it easier if you do not see `ABSL_INTERNAL_ATTRIBUTE_CAPTURED_BY(this)` outside abseil.

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