Custom main and global executors work hasn't passed Swift Evolution yet,
so we need to avoid leaking it as API until it does.
To that end, underscore all the things.
rdar://151147606
Remove the hacky support for mapping clocks to a Dispatch clock ID,
in favour of clocks publishing traits and having the Dispatch
executor select the clock on the basis of those traits.
Added an `-executor-factory` argument to the compiler to let you safely
specify the executors you wish to use (by naming a type that returns
them).
Also added some tests of the new functionality.
rdar://141348916
