[libc++] Make __debug_less::operator() constexpr

This is a followup to [1] which added a new `__debug_less::operator()` overload.
[2] added `_LIBCPP_CONSTEXPR_AFTER_CXX17` to the original
`__debug_less::operator()` between the time of writing [1] and landing it.  This
change adds `_LIBCPP_CONSTEXPR_AFTER_CXX17` to the new overload too.

[1] https://reviews.llvm.org/rL358423
[2] https://reviews.llvm.org/rL358252

Differential Revision: https://reviews.llvm.org/D60724

llvm-svn: 358725