[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.

从链接的 Stack Overflow 最小可工作示例开始，使用所述的 visibility flags 通过 Clang UBSan 重现 1.69.0 中的回归。确定该报告反映的是实际的未定义行为，还是 UBSan 的误报；完成此工作需要确认的诊断，以及有文档记录的修复或上游解决方案。

由索引模型根据 Issue 内容生成。