[Chrono][Darwin] Make steady_clock use CLOCK_UPTIME_RAW

Use CLOCK_UPTIME_RAW in case clock_gettime is available on Darwin.

On Apple platforms only CLOCK_UPTIME_RAW or mach_absolute_time are able
to time functions in the nanosecond range. Thus, they are the only
acceptable implementations of steady_clock.

Differential Revision: https://reviews.llvm.org/D27429

rdar://problem/29449467

llvm-svn: 291466