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.
Some conventions are measurably better then others, and this is exactly the case. I'm not going to argue this here anyway. 

Keep in mind that we even require specific style of braces in Phobos which is far more minor thing and, yes, 
"which one someone uses is their personal preference" is applicable (more or less) to any style requirement.