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`.