[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 commented:

Thank you for this! I only spotted two minor things, but otherwise think this LG.

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