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.
OK, 2 new comments :)

1. I think the clockArg should be by default normal, not by default coarse. We don't know what "other" clocks would represent on other platforms. The way the code is now, it by default selects coarse. I'm wondering actually if it shouldn't be a final switch, so any new clocks added force the developer to handle those cases specifically.
2. The static error message seems to focus only on COARSE, but it's possible all the clock defines may be differently named. I would change it to "Please handle all the clock types for this system."