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

[Gábor Horváth via cfe-commits]

Xazax-hun wrote:

> Given this is a keyword, maybe we should consider not deprecating that, and only deprecate things that actually conflict with identifiers. 

I would also be OK with keeping `this` as an argument to the attribute. I wonder what @AaronBallman 's take is on this. 

> 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.

Since most codebases are using these attributes behind a macro, the naive fix-its would not necessarily do the right thing. We can come up with some heuristics to try to infer the right macro to use but I am not sure if that meets the bar for a compiler warning fix it. 

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