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

[Hans Wennborg via cfe-commits]

zmodem wrote:

This breaks all existing code using `lifetime_capture_by(this)`, right?

Can you provide an example of how such code should be changed to build warning-free with Clang versions both before and after this change?

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