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

[Reid Kleckner via cfe-commits]

rnk wrote:

I added some reviewers to increase visibility. This warning is designed to avoid common pitfalls with shared objects and hidden visibility as I understand it, so I thought @maskray might be able to help with some ELF linker/loader insight as to whether this warning is valuable.

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