[Bug libstdc++/78595] Unnecessary copies in _Rb_tree

[ville.voutilainen at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=78595

--- Comment #4 from Ville Voutilainen <ville.voutilainen at gmail dot com> ---
(In reply to Jonathan Wakely from comment #3)

While I'm at it, I'll move the constraint to the return type.