DOSHeuristics DeadlineClock more explicit on 5.6

[rnro]

Motivation:

The type inference system isn't able to work out the default clock type in the DOSHeuristics init on Swift 5.6 so this change makes it more explicit

Modifications:

Add more explicit inits

Result:

The code should build on 5.6 and later

[Lukasa]

API breakage is failing compiling the old main.