[Regression 1.69.0] Clang UBSan triggers when std::string is used in notify()

We found that clang UBSan shows undefined behaviour when we use an option of type std::string with po::notify(). Please see https://stackoverflow.com/q/55064334/2375725 for more details and a minimal working example.
My colleague has already bisected the issue in [this comment](https://stackoverflow.com/questions/55064334/ubsan-boostprogram-options-with-stdstring?noredirect=1#comment96884751_55064334) and it seems to be related to activating ``-fvisibility-inlines-hidden`` and ``-fvisibility=hidden``.
Unfortunately we have no idea where to go from here...

We will also open a ticket at clang as we are really not sure whether this is a false positive in clang UBSan or actual undefined behaviour.

Start with the linked Stack Overflow minimal working example and reproduce the regression from 1.69.0 using Clang UBSan with the stated visibility flags. Determine whether the report reflects actual undefined behaviour or a UBSan false positive; completion requires a confirmed diagnosis and a documented fix or upstream resolution.

Vom Indexierungsmodell aus dem Issue-Text verfasst.