[libc++] Fix std::__debug_less in c++17.

b07b5bd7 adds a use of `__comp_ref_type.h` to `std::min`. When libc++ is built with `-D_LIBCPP_DEBUG=0`, this enables `std::__debug_less`, which is only marked constexpr after c++17.

`std::min` itself is marked as being `constexpr` as of c++14, so by extension, `std::__debug_less` should also be marked `constexpr` for the same versions so that `std::min` can use it. This change lowers the guard from `> 17` to `> 11`.

Reproducer in godbolt: https://godbolt.org/z/ans3TGsj8

```

constexpr int x() { return std::min<int>({1, 2, 3, 4}); }

static_assert(x() == 1);
```

Reviewed By: #libc, philnik, Quuxplusone, ldionne

Differential Revision: https://reviews.llvm.org/D118940