From #143177. This combines the summaries for the pre- and post C++ 11 `std::string` as well as `std::wstring`. In all cases, the data pointer is reachable through `_M_dataplus._M_p`. It has the correct type (i.e. `char*`/`wchar_t*`) and it's null terminated, so LLDB knows how to format it as expected when using `GetSummaryAsCString`.