The implementation is based off of std::bind_front. Since this is a
C++23 feature we use deducing this unconditionally.
PR libstdc++/108827
PR libstdc++/111327
libstdc++-v3/ChangeLog:
* include/bits/version.def (bind_back): Define.
* include/bits/version.h: Regenerate.
* include/std/functional (_Bind_back): Define for C++23.
(bind_back): Likewise.
* testsuite/20_util/function_objects/bind_back/1.cc: New test
(adapted from corresponding bind_front test).
* testsuite/20_util/function_objects/bind_back/111327.cc: Likewise.
ac1a399bf6