[Bug libstdc++/78134] set::set lower_bound() for transparent comparator returns const_iterator

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=78134

--- Comment #1 from Jonathan Wakely <redi at gcc dot gnu.org> ---
It's not returning S::const_iterator, if it was the assertion would pass
because S::iterator and S::const_iterator are the same type. The problem is
that it's returning _Rb_tree::iterator rather than wrapping it in a
set::iterator.