Detect libc++ std version in folly::chrono backports

Summary: [Folly] Detect libc++ std version in `folly::chrono` backports, since libc++ does not define the recommended feature macros.

Reviewed By: wez

Differential Revision: D14002296

fbshipit-source-id: 022085658aee3a453c5aea714f75f27013825c6f

folly/Chrono.h

@@ -31,7 +31,7 @@
  *  * std::chrono::round
  */
 
-#if __cpp_lib_chrono >= 201510 || _MSC_VER
+#if __cpp_lib_chrono >= 201510 || _LIBCPP_STD_VER > 14 || _MSC_VER
 
 namespace folly {
 namespace chrono {
@@ -150,7 +150,7 @@ template <
         detail::is_duration<To>::value &&
         !std::chrono::treat_as_floating_point<typename To::rep>::value>::type>
 constexpr To round(std::chrono::duration<Rep, Period> const& d) {
-  return detail::round_impl(d, folly::chrono::floor<To>(d));
+  return detail::round_impl(d, floor<To>(d));
 }
 
 //  mimic: std::chrono::round, C++17