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

[Aaron Ballman via cfe-commits]

https://github.com/AaronBallman approved this pull request.

LGTM aside from some coding style nits. Because AST printing is not part of the public interface for Clang, I don't think these changes need a release note.

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