Add ClockType enum to core.time for issue# 13433.

This adds an enum for indicating which type of clock to use when it's
appropriate for a time function to have multiple options for the source
clock. Currently, it only supports "normal" and "coarse," but we may add
more in the future, since some systems do support others (e.g
clock_gettime has several other options depending on the system).

This is in support of https://github.com/D-Programming-Language/phobos/pull/2584.
No. That's a typo. So, I don't know how the code is even compiling, since I didn't think that CLOCK_MONOTONIC_FAST was there except in the freebsd version of the header. Maybe it was one of the ones that was in the shared header but got deprecated.