[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
diff --git a/libcxx/include/algorithm b/libcxx/include/algorithm
index 244ae2d..326e566 100644
--- a/libcxx/include/algorithm
+++ b/libcxx/include/algorithm
@@ -785,6 +785,7 @@
     __debug_less(_Compare& __c) : __comp_(__c) {}
 
     template <class _Tp, class _Up>
+    _LIBCPP_CONSTEXPR_AFTER_CXX17
     bool operator()(const _Tp& __x,  const _Up& __y)
     {
         bool __r = __comp_(__x, __y);