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

[Aaron Ballman via cfe-commits]

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

LGTM aside from the comment nit from @ChuanqiXu9 

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