[clang] [Clang] Reland "Diagnose UB and emit error when identifier has both internal and external linkage" (PR #193567)

[Aaron Ballman via cfe-commits]

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

LGTM aside from some minor nits with the Clang version number; but please wait for sign-off from @Endilll before landing.

https://github.com/llvm/llvm-project/pull/193567