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.
Well, and the fact that in order to remain binary-compatible, new values would have to go on the end, which looks messy when Linux values aren't in the same order as FreeBSD. But of course, these are the private, non-documented ones.