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.
Okay I'll clarify my reservations:

When I read "Most programs should just use $(D ClockType.normal)" I expect either period and I'm done reading or some _reason_ for it being default. Instead I see long sentances basically describing in painful detail how deafult arguments work.

Then comes the reason for _other_ clock types and I concur with Martin, let's make it the first paragraph.

"The multitude of clock types are given just so that programs have access to  the some of the other that the underlying system provides" 
A long way to tell:
 "Other clock types correspond to the more specific clocks that the OS may provide"

While I'm obviously biased, I'd say there needs to be x2 less text, covering only key facts
Docs are read very often, and more typically in a "how to get shit done in 5 minutes" mode rather then "let me read this module's docs tonight so I understand it better".