Include C++17 type_traits and C++20 chrono calendars

[trxcllnt]

This PR and its sister PR in ogiroux/freestanding encapsulate the changes necessary to use the C++17 type_traits and C++20 chrono calendars in cudf with the -std=c++14 flag. See the notes in that PR for a description of these commits.

[brycelelbach]

First, we created an abstraction for hooking in our own clock backend, the external clock API. But Paul found that didn't work when he needed the C++20 chrono stuff, which internally depends on the clock implementation. So he moved the clocks into the upstream libc++ headers, which defeats the purpose of the external clock API. I think we should take a step back and think about how to fix the external clock API/make it work for what Paul's doing, not just put our implementation into the upstream libc++ headers.