[Bug libstdc++/117655] std::string::swap() could be much faster and smaller

[redi at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=117655

--- Comment #3 from Jonathan Wakely <redi at gcc dot gnu.org> ---
(In reply to Mathias Stearn from comment #2)
> It looks like a similar optimization would make sense for
> operator=(string&&): https://godbolt.org/z/Wo19fjKeK.

I think this is missing the fix up, although it still beats what we have on
trunk today:
_M_data(_M_data() == b._M_local_data() ? _M_local_data() : _M_data());