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.
Sure, by why bother? If you want that, just use the coarse clock. The only reason that `second` exists at all is to support FreeBSD's `CLOCK_SEC`, and it seemed like it would be reasonable to fake it on other platforms so that it could be cross-platform. We'd be faking the whole thing for the monotonic clock. We wouldn't actually be adding support for any additional underlying clocks.