[clang] Warn when unique objects might be duplicated in shared libraries (PR #117622)

[Devon Loehr via cfe-commits]

DKLoehr wrote:

Addressed feedback, responded where appropriate. I've also done some digging into how the warning should work on Windows: it seems like it'll be very straightforward (just replace "has hidden visibility" with "doesn't have a dll import/export attribute"), but I'll leave that for a future PR to keep each the PRs focused.

https://github.com/llvm/llvm-project/pull/117622