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

[Hans Wennborg via cfe-commits]

https://github.com/zmodem commented:

Some scattered thoughts:

- What about code that's not going into a shared library. Should we disable the warning then, or is the idea that the user does not enable the warning flag in that case?

- The new warning is DefaultIgnore. Do you figure it's too noisy to be on by default, or to be in some category like -Wextra? If it's just DefaultIgnore, users are unlikely to find it.

> The check is also disabled inside templates, since it can give false positives if a template is never instantiated.

Would warning during instantiation work, or is that also too noisy?

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