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, I also find UFCS incredibly backwards, and that seems to go against what most people think - though in that case I think that it's clearly a case of my having done a lot more functional programming that the average programmer, so I'm very used to function chaining operating right-to-left instead of right-to-left, whereas the folks who do imperative programming almost exclusively aren't.