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.
I wonder if this jives with the description that:

If the function in question does not support the given clock (or it does on some systems but not the current system), then ClockType.normal is used.

I understand what you are trying to do, but I wonder if it should just be in a unit test instead of in the main code, so the developers are aware of it, but it doesn't impede adoption on a new platform.