Using single-threaded concurrency was a temporary solution, now that the task-to-thread model actually supports multiple threads, let's switch off of it. Instead, let's introduce a "global executor none" option (implicitly set under the task-to-thread model) to denote that the concurrency model is not using a global executor.
rdar://99448771
A task can be in one of 4 states over its lifetime:
(a) suspended
(b) enqueued
(c) running
(d) completed
This change provides priority inversion avoidance support if a task gets
escalated when it is in state (a), (c), (d).
Radar-Id: rdar://problem/76127624
