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.
Yeah. That matches my understanding, and I agree that clocks shouldn't be mixed, but mixing the coarse and normal clocks should mostly work (particularly if you're dealing with values grabbed at least a millisecond or two apart), but other clocks would be a different story entirely, since their values wouldn't even necessarily mean the same thing. I'll add a note about not mixing clocks though.