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.
Ah, I see what you mean now. I intended for it to be a static assertion, but it's a regular assertion. It's supposed to fail at compile time. That's what we typically do with the else branch of a version block in druntime and Phobos. I just screwed up and forgot the `static`.