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

facebook/folly,facebook/folly,facebook/folly,facebook/folly,facebook/folly