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.
There are plenty of cases in druntime/Phobos where they haven't been rewritten (e.g. all of the aliases in std.datetime still use the older syntax, and a quick grep shows that there are quite a few places in other modules as well). But I see no reason to stop using the older syntax unless we're actually going to get rid of it, and AFAIK, that's not the plan.