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

[Reid Kleckner via cfe-commits]

rnk wrote:

+1 to separating implementation from enablement. Enabling the warning by default will predictably generate pushback and folks may ask to revert, and it's a good best practice to be able to easily do that while leaving the implementation in place.

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