libstdc++: Fix std::ranges::to errors
Fix some errors that Patrick noticed, and remove a #if 0 group that I
didn't mean to leave in the file.
libstdc++-v3/ChangeLog:
* include/std/ranges (__detail::__toable): Fix incorrect use of
_Range instead of _Cont.
(__detail::_ToClosure, __detail::_ToClosure2): Add missing
constexpr specifier on constructors.
* testsuite/std/ranges/conv/1.cc (_Cont, _Cont2, _Cont3): Remove
unnecessary begin() and end() members.
(test_constexpr): New function to check range adaptors are
usable in constant expressions.