Remove the disable of availability and mark methods as available to c…

…lock/instant/duration (apple#165)

* Remove the disable of availability and mark methods as available to clock/instant/duration

* Rework availability to be more permissive for tests

* Use hand coded values instead of defines (only available via Darwin versions of Dispatch)

* Adjust additional test availability

* Correct debounce test availability guard

* Update build instructions to indicate Xcode requirements

* Ensure the clock property of the validation diagram is only available for targets that can support it

Update availability to be in sync with Clock/Instant/Duration (apple#159

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Add shims and adjust availability to allow for swift 5.6 toolchains t…

…o build until source stability is achieved (apple#100)

* Add shims and adjust availability to allow for swift 5.6 toolchains to build until source stability is achieved

* Restore availability again