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

[Gábor Horváth via cfe-commits]

https://github.com/Xazax-hun requested changes to this pull request.

Could you elaborate on the problem? 

@NeKon69 suggested a transition path on the original PR, and the old syntax is still supported, one can just disable the deprecation warning in the worst case.

I am not sure how much more we can do to support the transition path. Maybe if that suggested approach does not work we could add a feature test macro? 

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