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.
Honestly, I think that insisting on one alias syntax over the other is like insisting on `while(1)` over `for(;;)` - or vice versa. They do the same thing, and which one someone uses is their personal preference. It has zero effect on the meaning of the code. If we decide to actually deprecate and remove the older syntax, then fine, but as long as we have the older one, I'm going to continue to use it.