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.
Just a thought, I think saying everywhere what Clock.currTime doesn't support is tedious. Why not just say what Clock.currTime *does* support, and say at the top "Clock.currTime only supports a few clocks, noted where applicable"?