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

[Hans Wennborg via cfe-commits]

zmodem wrote:

For reference, this is what the fix in Abseil looks like (thanks Utkarsh!): https://github.com/abseil/abseil-cpp/commit/0784689a65b4ea1329fbc5174b8a57c88ec796a9 Is that better than what we had before? Not sure, but I'll stop complaining :)

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