[clang] [clang-tools-extra] [clang] Refactor `clang::Linkage` (PR #71049)

[Chuanqi Xu via cfe-commits]

https://github.com/ChuanqiXu9 approved this pull request.

LGTM. I didn't understand the full context in the private chat.

https://github.com/llvm/llvm-project/pull/71049