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

[Corentin Jabot via cfe-commits]

cor3ntin wrote:

Given `this` is a keyword, maybe we should consider not deprecating that, and only deprecate things that actually conflict with identifiers. Or deprecate it separately with a less aggressive timeline.

Also. When you merge a PR @Xazax-hun @NeKon69 , can you make sure the description reflects what's the PR is doing? i.e you introduced 3 new attributes, that should be in the description.

I also do not understand why there is no fix-it for this very disruptive depreciation diagnostic? That certainly would have mitigated somewhat the impact of the change.

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