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

[Aaron Ballman via cfe-commits]

AaronBallman wrote:

To be clear, if deprecating these attributes really is an intractably painful thing without a dedicated diagnostic group, I can hold my nose so that users can have a more reasonable experience, but I would argue that it should not be a precedent for other attributes or maybe is a signal that we need a tighter community policy on design review for attributes before we accept them.

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