The new intrinsic, exposed via static functions on Task<T, Never> and
Task<T, Error> (rethrowing), begins an asynchronous context within a
synchronous caller's context. This is only available for use under the
task-to-thread concurrency model, and even then only under SPI.
A copy of the older Concurrency sources was just added to BackDeployConcurrency,
replacing the `add_subdirectory(../Concurrency)` trick it was using previously.
These need to be adjusted to work with the threading library. Fortunately, the
necessary adjustments are the same as those required for the existing
Concurrency library, give or take.
rdar://90776105
