[clang] Revert "[clang][Sema] Deprecate `global`/`unknown` in `lifetime_capture_by`" (PR #208691)

[Gábor Horváth via cfe-commits]

Xazax-hun wrote:

To be absolutely clear, the longer we wait the more painful the migration will be for folks. So I'd strongly prefer not to revert this unless there is a very strong reason to do it. And I had the impression we already have multiple migration paths, the conditional macro or disabling the deprecation warning. 

https://github.com/llvm/llvm-project/pull/208691