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.
> but I really don't see any point in forcing folks to use the newer syntax unless we're actually going to get rid of the older syntax

One word: Consistency. Even setting aside the fact that almost everybody finds the new syntax more natural (do you seriously find normal assignments to backwards?), having a codebase follow a consistent set of formatting guidelines reduces the mental overhead when jumping between different places.