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.
We can go that route. I do like the idea of just supporting them all, but we also clearly don't really know what they do, and I have no idea how many people are going to care. So, if we want to just wait until some asks for them (at which point we can ask _them_ what they're supposed to do), then that's fine.