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.
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.
Without this hack the call will leave a stack frame around (not tail
call optimized) and blow the stack if we call switch_task often enough.
Ideally, clang would emit this call as `musttail` but currently it does
not.
rdar://76652421
