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,
instead of disabling availability checking.
The "#if compiler(>=5.3) && $AsyncAwait" checks were necessary for
staging in concurrency in Swift 5.5. At this point, it's safe to assume
that any compiler that tries to read a generated Swift interface file will
support concurrency, so we can stop emitting these guards.
1. Removes gating on -enable-experimental-concurrency.
2. Updates eff. prop tests to remove experimental flag,
and also adjusts some tests slightly to avoid things
that are still behind that flag.
