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, I'm ready to say this is good to go, but I'm curious. I see some potential, very slight but still there, that these tests may fail, based on the usage of the system being tested on (which would be a false failure). Is there a good reason to have these tests?