[libc++] Avoid including things that require a filesystem in filesytem_clock.cpp

[libc++] Avoid including things that require a filesystem in filesytem_clock.cpp

The filesystem clock implementation should be available regardless of
whether a proper filesystem is available on the platform, so it makes
sense to try and avoid including things that are inherently filesystem-y
in the implementation of filesystem clock.

Differential Revision: https://reviews.llvm.org/D154390