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.
I'm thinking we might just want to not support virtual and prof until we know what they are for and someone needs them.

I saw this on open bsd forum, dated pretty recently. The commentary on what FreeBSD does with these doesn't seem very reassuring: http://openbsd.7691.n7.nabble.com/implement-CLOCK-VIRTUAL-and-CLOCK-PROF-td257532.html

I also just realized, `virtual` was almost a keyword, and is a keyword in many languages related to D. We probably shouldn't use that as an identifier anywhere.