Instead, use the `%target-swift-5.1-abi-triple` substitution to compile the tests
for deployment to the minimum OS versions required for use of _Concurrency APIs.
We won't look for async functions that can't be used due to availability
unless the availability checking is disabled. Need to disable the
availability checking due to the minimum deploy target being too low for
concurrency.
The concurrency runtime now deploys back to macOS 10.15, iOS 13.0, watchOS 6.0, tvOS 13.0, which corresponds to the 5.1 release of the stdlib.
Adjust macro usages accordingly.
Rather than blanket-disabling concurrency tests when we aren't using a
just-built concurrency library, enable them whenever we have a
suitable concurrency runtime, either just-built, in the OS, or via the
back-deployment libraries.
These tests require a newer runtime and fail on older OSes:
test/Concurrency/Runtime/actor_detach.swift
test/Concurrency/Runtime/actor_init.swift
test/Interpreter/rdar80847020.swift
test/stdlib/Reflection_objc.swift
