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.
* Enable running tests from test/Concurrency/ directory in freestanding/minimal presets
* Mark failing Concurrency tests as XFAIL/SKIP on freestanding/minimal
These were previously disabled due to the UB that caused the tests to
fail. Now that this has been fixed, restore these tests to running
state in line with the other targets.
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.
