[clang] [clang][AST] fix ast-print of `extern <lang>` with >=2 declarators (PR #93131)

[Artem Yurchenko via cfe-commits]

temyurchenko wrote:

> > What should be the next step following the approvals?
> 
> If you do not have 'squash and merge' rights here, one of us can do it for you once testing completes.

I don't have them, would be glad if you could do that!

https://github.com/llvm/llvm-project/pull/93131