[PATCH] PR libstdc++/59439 optimize uses of classic ("C") std::locale

[Florian Weimer]

* Jonathan Wakely:

> Does anybody see any problems with this change?

Can you put a debug assert into _M_add_reference and
_M_remove_reference, to check that they are never called on an
_S_classic object?  Or put the check directly into those functions?

Thanks,
Florian