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

[via cfe-commits]

NeKon69 wrote:

> This is still marked as a draft but seems ready for review?

I guess? Though, I am still not 100% certain we want to go with this approach (I would like to have some confirmation from other people), and that's also the reason why I didn't change `clang/docs/ReleaseNotes.rst` yet.

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